Combustion control system of internal combustion engine

ABSTRACT

In a combustion control system of an engine employing an exhaust purifying device, a control unit determines, based on an operating condition of the exhaust purifying device, whether a request for an exhaust temperature rise is present. The control unit executes, by way of fuel injection control in presence of the request of the exhaust temperature rise, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, and the preliminary combustion occurs near top dead center on a compression stroke, and the main combustion initiates after completion of the preliminary combustion. The control unit executes a fail-safe process that an injection timing of main fuel injection is phase-advanced, when a predicted temperature value of the exhaust purifying device exceeds a predetermined threshold value.

TECHNICAL FIELD

The present invention relates to a combustion control system of aninternal combustion engine, and specifically to the improvement of acombustion control technology for an internal combustion engine capableof executing a split fuel injection mode in presence of a request for anexhaust temperature rise such as a catalyst temperature rise neededduring cold engine operation.

BACKGROUND ART

In recent years, there have been proposed and developed variouscombustion control technologies in fuel injection engine systems capableof executing a split fuel injection mode suitable for a rapid exhausttemperature rise, for example in presence of a request for rapidactivation of catalyst. One such combustion control system with splitfuel injection system interaction has been disclosed in Japanese PatentProvisional Publication No. 2000-320386 (hereinafter is referred to as“JP2000-320386”). In the Diesel-engine fuel injection control systemdisclosed in JP2000-320386, in presence of a request for a catalyst bedtemperature rise, the fuel injection control system executes a splitfuel injection mode in which a basic amount of fuel, determined based ona required engine torque, is sprayed or injected thrice separately byway of three-split fuel injection near top dead center (TDC) on thecompression stroke. During the split fuel injection mode, if necessary,the fuel injection amount is increasingly compensated for.

SUMMARY OF THE INVENTION

However, during the split fuel injection mode executed by the fuelinjection system disclosed in JP2000-320386, when taking into accountthe time interval (the fuel-injector pulse width or time duration)between the end of injection of the early injected fuel portion of thethree-split fuel injection and the start of injection of theintermediately injected fuel portion, and the time interval between theend of injection of the intermediately injected fuel portion and thestart of injection of the lastly injected fuel portion, these timeintervals are set to be very short. Owing to the very short injectiontime intervals of split fuel injection, combustion of the early injectedfuel portion, combustion of the intermediately injected fuel portion,and combustion of the lastly injected fuel portion tend to continuouslyoccur. In more detail, the intermediate-injection fuel portion would besprayed into the flame of the early-injection fuel portion, and then thelast-injection fuel portion would be sprayed into the flame of theearly-injection fuel portion and/or the flame of theintermediate-injection fuel portion. As a result of this, combustion ofthe intermediately and lastly injected fuel portions may be mainlycomposed of diffusion combustion rather than pre-mixed combustion.Assuming that an air/fuel mixture ratio (A/F ratio) is changed to rich(λ<1) under such a combustion condition (when a ratio of diffusioncombustion to premixed combustion is great), there is an increasedtendency for exhaust emissions of smoke and particulates to remarkablyincrease.

Accordingly, it is an object of the invention to provide a combustioncontrol system of an internal combustion engine capable of realizingoptimum combustion without increased exhaust emissions of smoke andparticulates even when enriching an air/fuel mixture ratio (A/F ratio)in presence of a request for an exhaust temperature rise, andadditionally to provide the combustion control system having a fail-safefunction that prevents an exhaust purifying device (an exhaust purifyingcatalyst) from being damaged owing to an excessive exhaust temperaturerise when intentionally rising the exhaust temperature.

In order to accomplish the aforementioned and other objects of thepresent invention, a combustion control system of an internal combustionengine employing an exhaust purifying device in an exhaust passage,comprises sensors that detect operating conditions of the engine, acontrol unit being configured to be electronically connected to thesensors, for combustion control and fail-safe purposes, the control unitcomprising a processor programmed to perform the following, estimatingan operating condition of the exhaust purifying device, determining,based on the operating condition of the exhaust purifying device,whether a request for a rise in an exhaust temperature is present,executing, by way of fuel injection control in presence of the requestfor the exhaust temperature rise, a split retard combustion mode inwhich a main combustion needed to produce a main engine torque and atleast one preliminary combustion occurring prior to the main combustionare both achieved and additionally the preliminary combustion takesplace near top dead center on a compression stroke and additionally themain combustion initiates after the preliminary combustion has beencompleted, predicting a temperature of the exhaust purifying device todetermine a predicted temperature value, and executing a fail-safeprocess according to which an injection timing of main fuel injectionfor the main combustion is compensated for in a timing-advancedirection, when the predicted temperature value exceeds a predeterminedtemperature threshold value.

According to another aspect of the invention, a combustion controlsystem of an internal combustion engine employing an exhaust purifyingdevice in an exhaust passage, comprises sensor means for detectingoperating conditions of the engine, a control unit being configured tobe electronically connected to the sensor means, for combustion controland fail-safe purposes, the control unit comprising means for estimatingan operating condition of the exhaust purifying device, means fordetermining, based on the operating condition of the exhaust purifyingdevice, whether a request for a rise in an exhaust temperature ispresent, means for executing, by way of fuel injection control inpresence of the request for the exhaust temperature rise, a split retardcombustion mode in which a main combustion needed to produce a mainengine torque and at least one preliminary combustion occurring prior tothe main combustion are both achieved and additionally the preliminarycombustion takes place near top dead center on a compression stroke andadditionally the main combustion initiates after the preliminarycombustion has been completed, means for predicting a temperature of theexhaust purifying device to determine a predicted temperature value, andmeans for executing a fail-safe process according to which an injectiontiming of main fuel injection for the main combustion is compensated forin a timing-advance direction, when the predicted temperature valueexceeds a predetermined temperature threshold value.

According to a further aspect of the invention, a method of executing afail-safe function for an exhaust purifying device disposed in anexhaust passage of an internal combustion engine capable of recoveringan operating condition of the exhaust purifying device, the methodcomprises estimating the operating condition of the exhaust purifyingdevice, determining, based on the operating condition of the exhaustpurifying device, whether a request for a rise in an exhaust temperatureis present, executing, by way of fuel injection control in presence ofthe request for the exhaust temperature rise, a split retard combustionmode in which a main combustion needed to produce a main engine torqueand at least one preliminary combustion occurring prior to the maincombustion are both achieved and additionally the preliminary combustiontakes place near top dead center on a compression stroke andadditionally the main combustion initiates after the preliminarycombustion has been completed, predicting a temperature of the exhaustpurifying device to determine a predicted temperature value, andexecuting a fail-safe process according to which an injection timing ofmain fuel injection for the main combustion is compensated for in atiming-advance direction, when the predicted temperature value exceeds apredetermined temperature threshold value.

According to a still further aspect of the invention, a method ofexecuting a fail-safe function for an exhaust purifying device includinga NOx trap catalyst that traps nitrogen oxides contained in exhaustgases when an exhaust air-fuel mixture ratio is lean, and a particulatefilter that accumulates particulate matter contained in the exhaustgases and is disposed downstream of the NOx trap catalyst, both disposedin an exhaust passage of an internal combustion engine capable ofrecovering an operating condition of the exhaust purifying device, themethod comprises disposing a first exhaust temperature sensor downstreamof the NOx trap catalyst and upstream of the particulate filter,disposing a second exhaust temperature sensor downstream of theparticulate filter, predicting a temperature of the NOx trap catalystbased on an exhaust temperature detected by the first exhausttemperature sensor, predicting a temperature of the particulate filterbased on both of the exhaust temperature detected by the first exhausttemperature sensor and an exhaust temperature detected by the secondexhaust temperature sensor, estimating the operating condition of theexhaust purifying device, determining, based on the operating conditionof the exhaust purifying device, whether a request for a rise in anexhaust temperature is present, executing, by way of fuel injectioncontrol in presence of the request for the exhaust temperature rise, asplit retard combustion mode in which a main combustion needed toproduce a main engine torque and at least one preliminary combustionoccurring prior to the main combustion are both achieved andadditionally the preliminary combustion takes place near top dead centeron a compression stroke and additionally the main combustion initiatesafter the preliminary combustion has been completed, and executing afail-safe process according to which an injection timing of main fuelinjection for the main combustion is phase-advanced, when the at leastone of the NOx-trap-catalyst temperature predicted and theparticulate-filter temperature predicted exceeds a predeterminedtemperature threshold value.

The other objects and features of this invention will become understoodfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram illustrating an embodiment of acombustion control system of an internal combustion engine.

FIG. 2 is a flow chart showing an exhaust emission control routineexecuted by the combustion control system of the embodiment.

FIG. 3 is a flow chart showing a first subroutine (diesel particulatefilter (DPF) regeneration mode) for exhaust emission control.

FIG. 4 is a flow chart showing a second subroutine (sulfur poisoningrelease mode) for exhaust emission control.

FIG. 5 is a flow chart showing a third subroutine (rich spike mode) forexhaust emission control.

FIG. 6 is a flow chart showing a fourth subroutine (melting lossprevention mode) for exhaust emission control.

FIG. 7 is a flow chart showing a fifth subroutine (order-of-prioritydecision routine in presence of a request for DPF regeneration) forexhaust emission control.

FIG. 8 is a flow chart showing a sixth subroutine (order-of-prioritydecision routine in presence of a request for sulfur poisoning release)for exhaust emission control.

FIG. 9 is a flow chart showing a seventh subroutine (DPF-regenerationrequest indicative flag rq-DPFFLAG setting routine) for exhaust emissioncontrol.

FIG. 10 is a flow chart showing an eighth subroutine (sulfur poisoningrelease request indicative flag rq-desulFLAG setting routine) forexhaust emission control.

FIG. 11 is a flow chart showing a ninth subroutine (rich spike requestindicative flag rq-spFLAG setting routine) for exhaust emission control.

FIG. 12 is a flow chart showing a tenth subroutine (warm-up promotionmode) for exhaust emission control.

FIGS. 13A-13B are time charts showing a fuel injectionpattern/combustion type in a first comparative example {circle over(1)}.

FIGS. 14A-14B are time charts showing a fuel injectionpattern/combustion type in a second comparative example {circle over(2)}.

FIGS. 15A-15B are time charts showing a fuel injectionpattern/combustion type in a split retard combustion mode {circle over(3)} of the system of the embodiment.

FIG. 16 is an explanatory view showing comparison results concerningexhaust-emissions conditions, namely exhaust temperature, smokeconcentration, hydrocarbons (HCs) concentration, respectively in case ofthe first comparative example {circle over (1)}, the second comparativeexample {circle over (2)}, and the split retard combustion mode {circleover (3)} of the system of the embodiment.

FIGS. 17A-17D are characteristic curves respectively showing the exhausttemperature characteristic, the smoke concentration characteristic, thecarbon monoxide concentration characteristic, and the hydrocarbonsconcentration characteristic, during the main injection period.

FIG. 18 is a characteristic map showing a desired fuel injection timingsuitable for preliminary combustion.

FIG. 19 is a characteristic map showing a desired fuel injectionquantity for preliminary combustion.

FIG. 20 is a characteristic map showing a desired fuel injection timingsuitable for main combustion.

FIGS. 21A-21B are time charts showing a fuel injectionpattern/combustion type in another split retard combustion mode (multipreliminary combustion plus main combustion mode) {circle over (4)} ofthe system of the embodiment.

FIG. 22 is a flow chart showing aswitching-to-split-retard-combustion-mode routine.

FIG. 23 is a characteristic diagram showing the relationship between aparticulate-matter accumulation quantity (PM quantity) and a desired λduring DPF regeneration.

FIG. 24 is a characteristic diagram showing the relationship between aDPF-regeneration and sulfur-poisoning-release enabling region (DPF, SOxregeneration enabling region) and a DPF-regeneration andsulfur-poisoning-release disabling region (DPF, SOx regenerationdisabling region).

FIG. 25 is a flow chart showing air quantity control containing enginetorque compensation.

FIG. 26 is a flow chart showing fuel injection quantity control.

FIG. 27 is a characteristic map showing the relationship among a basictorque correction factor Ka1, desired λ, and engine speed Ne.

FIG. 28 is a preprogrammed torque correction factor Ka2 versus maininjection timing characteristic map.

FIG. 29 is a preprogrammed torque correction factor Ka3 versus enginecoolant temperature Tw characteristic map.

FIG. 30 is a flow chart used for catalyst temperature estimation.

FIG. 31 is a flow chart showing used for DPF temperature estimation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, particularly to FIG. 1, the combustioncontrol system of the embodiment is exemplified in a turbocharged Dieselengine 1 equipped with an exhaust purifying device containing a NOx trapcatalyst 13 and a diesel particulate filter (DPF) 14. As seen from thesystem block diagram of FIG. 1, an air compressor of a variable-nozzleturbocharger 3 is disposed in an intake-air passage 2 of Diesel engine 1for introducing air for combustion through an inter-cooler 4 and anelectronically-controlled throttle valve 5 via a collector 6 into eachindividual engine cylinder at a pressure in excess of that which can beobtained by natural aspiration. The throttle opening of throttle valve 5is electronically controlled by means of a throttle actuator attached tothrottle valve 5 and driven responsively to a control signal from anelectronic engine control unit 20 (described later). Fuel is supplied bymeans of a common-rail type fuel injection system. High-pressure fuel,pressurized by a high-pressure fuel pump 7, is supplied to a common rail8. High-pressure fuel is directly injected from a fuel injector valve(simply, a fuel injector) 9 of each engine cylinder into a combustionchamber. An air-fuel mixture of the air introduced into the combustionchamber and the fuel injected into the same combustion chamber is burnedby way of compression ignition. On the exhaust stroke, exhaust gas flowsinto an exhaust passage 10. Part of the exhaust gas is sent back throughengine 1 by way of an exhaust-gas-recirculation (EGR) system. As clearlyshown in FIG. 1, the EGR system is comprised of an EGR passage 11 and anEGR control valve 12. For the purpose of EGR addition and the reducedamount of NOx, part of the exhaust gas is re-circulated into the intakemanifold side of the induction system of engine 1 through EGR passage 11and EGR control valve 12. In the same manner as throttle valve 5, theopening of EGR control valve 12 is electronically controlled by means ofan EGR valve actuator attached to EGR control valve 12 and drivenresponsively to a control signal from control unit 20. An exhaust-gasturbine of variable-nozzle turbocharger 3 is disposed in exhaust passage10. The remaining part of the exhaust gas is used to drive theexhaust-gas turbine of variable-nozzle turbocharger 3. As shown in FIG.1, for the purpose of exhaust gas purification, NOx trap catalyst 13 isdisposed in exhaust passage 10 and located downstream of the exhaust-gasturbine of turbocharger 3. When the exhaust A/F ratio is lean (λ>1), NOxtrap catalyst 13 operates to capture or trap or adsorb nitrogen oxides(NOx). Conversely when the exhaust A/F ratio is rich (λ<1), NOx trapcatalyst 13 operates to release NOx. NOx trap catalyst 13 is alsoequipped with an oxidation catalyst (precious metal) for oxidizing theincoming exhaust gas composition such as hydrocarbons (HCs) and carbonmonoxide (CO).

Additionally, diesel particulate filter (DPF) 14 is disposed in exhaustpassage 10 and located downstream of NOx trap catalyst 13, SO as toaccumulate particulate matter (PM) contained in the exhaust gases. DPF14 is also equipped with an oxidation catalyst (precious metal) foroxidizing the incoming exhaust gas composition such as HCs and CO.

As shown in FIG. 1, control unit 20, which is incorporated in the systemof the embodiment for combustion control of engine 1, generallycomprises a microcomputer. Control unit 20 includes an input/outputinterface (I/O), memories (RAM, ROM), and a microprocessor or a centralprocessing unit (CPU). The input/output interface (I/O) of control unit20 receives input information from various engine/vehicle sensors,namely an engine speed sensor 21, an accelerator opening sensor (or anaccelerator position sensor) 22, an airflow meter 23, and an enginetemperature sensor (an engine coolant temperature sensor) 24. Enginespeed sensor 21 detects the engine speed Ne, accelerator opening sensor22 detects the accelerator opening APO, airflow meter 23 detects theactual intake-air quantity Qac of air drawn into the engine, and enginecoolant temperature sensor 24 detects the engine coolant temperature Tw.

The input/output interface (I/O) of control unit 20 also receives inputinformation from an upstream exhaust temperature sensor (1^(st) exhausttemperature sensor) 29 _(T1), an exhaust pressure sensor 26, adownstream exhaust temperature sensor (2^(nd) exhaust temperaturesensor) 29 _(T2), and an A/F ratio sensor 28. Upstream exhausttemperature sensor 29 _(T1) is screwed into exhaust passage 10 andlocated downstream of NOx trap catalyst 13 and upstream of DPF 14 fordetecting the exhaust temperature just downstream of NOx trap catalyst13. Exhaust pressure sensor 26 is screwed into exhaust passage 10 andlocated downstream of upstream exhaust temperature sensor 29 _(T1) andupstream of DPF 14 for detecting the exhaust pressure. Downstreamexhaust temperature sensor 29 _(T2) is screwed into exhaust passage 10and located downstream of DPF 14 for detecting the exhaust temperaturejust downstream of DPF 14. In the system of the shown embodiment,upstream exhaust temperature sensor 29 _(T1) is provided to estimate acatalyst bed temperature (simply, a catalyst temperature, which will bedescribed later by reference to the temperature estimation routine ofFIG. 30), based on the exhaust temperature sensed just downstream of NOxtrap catalyst 13, since it is difficult to directly detect the catalysttemperature of NOx trap catalyst 13. Upstream and downstream exhausttemperature sensors 29 _(T1) and 29 _(T2) are provided to estimate theDPF temperature (which will be described later by reference to thetemperature estimation routine of FIG. 31), based on the exhausttemperatures sensed upstream and downstream of DPF 14, since it isdifficult to directly detect the DPF temperature of DPF 14.

Within control unit 20, the central processing unit (CPU) allows theaccess by the I/O interface of input informational data signals from thepreviously-discussed engine/vehicle sensors 21-24, 26, 28, 29 _(T1), and29 _(T2). The CPU of control unit 20 is responsible for carrying theengine control program stored in memories and is capable of performingnecessary arithmetic and logic operations containing combustion controlmanagement processing, that is, electronic throttle control achievedthrough the throttle actuator of throttle valve 5, electronic fuelinjection control (fuel injection timing control and fuel injectionquantity control both achieved by the injectors of the electronic fuelinjection system), EGR control achieved by the EGR valve actuator of EGRcontrol valve 12, and exhaust emission control (described later).Computational results (arithmetic calculation results), that is,calculated output signals are relayed through the output interfacecircuitry of control unit 20 to output stages, namely the throttleactuator of throttle valve 5, the electromagnetic solenoids of injectors9, and the EGR valve actuator of EGR control valve 12.

Hereunder described in detail is the emission control executed withincontrol unit 20. Actually, the combustion control system of theembodiment achieves the exhaust emission control comprised of DPFregeneration, sulfur poisoning release, and NOx desorption-purification(or NOx trap catalyst regeneration, simply, NOx regeneration). Duringthe DPF regeneration mode, the particulate matter (PM) accumulated inDPF 14 is burned and removed from DPF 14. During the NOx regeneration,NOx trapped is desorbed from NOx trap catalyst 13. During the sulfurpoisoning release mode, sulfur oxides (SOx) trapped by NOx trap catalyst13 is desorbed therefrom.

FIGS. 2 through 12 show the flow charts concerning the exhaust emissioncontrol executed within the processor of control unit 20 of thecombustion control system of the embodiment.

Referring now to FIG. 2, there is shown the exhaust emission controlroutine, which is executed as time-triggered interrupt routines to betriggered every predetermined time intervals.

At step S1, input information (Ne, APO, Qac, Tw, the catalysttemperature, exhaust pressure at the inlet of DPF 14, DPF temperature,and exhaust A/F ratio at the outlet of DPF 14) from the engine/vehiclesensors 21-24, 26, 28, 29 _(T1), and 29 _(T2) is read. As previouslydescribed, the catalyst temperature of NOx trap catalyst 13 is estimatedbased on the exhaust temperature detected by upstream exhausttemperature sensor 29 _(T1), whereas the DPF temperature is estimatedbased on the exhaust temperatures detected by upstream and downstreamexhaust temperature sensors 29 _(T1) and 29 _(T2). One method toestimate the catalyst temperature of NOx trap catalyst 13 and the DPFtemperature will be described later in reference to the flow chartsshown in FIGS. 30 and 31.

At step S2, a check is made to determine whether NOx trap catalyst 13 isnot yet warmed up (deactivated). When the catalyst temperature is lessthan or equal to a predetermined catalyst activation temperature T5, NOxtrap catalyst 13 is not yet warmed up and thus the routine proceeds fromstep S2 of FIG. 2 to step S1001 of the warm-up promotion control routineshown in FIG. 12. Conversely when the catalyst temperature is greaterthan the predetermined catalyst activation temperature T5, NOx trapcatalyst 13 has already been warmed up and activated and thus theroutine proceeds from step S2 to step S3.

At step S3, a NOx accumulation quantity (simply, NOx quantity) ofnitrogen oxides trapped and accumulated by NOx trap catalyst 13 isdetermined. For instance, the NOx quantity can be calculated orestimated based on an integrated value of engine speeds. The method toestimate the NOx quantity based on the integrated value of engine speedsis conventional and forms no part of the present invention, typicaldetails of such accumulated NOx quantity estimation method being setforth, for example, in U.S. Pat. No. 5,473,887, corresponding toJapanese Patent No. 2600492, the teachings of which are herebyincorporated by reference. Instead of using the integrated value ofengine speeds, the traveling distance may be used for NOx quantityestimation. Assuming that the integrated value of engine speeds is usedfor NOx quantity estimation, the integrated value must be reset to “0”at the point of time when NOx desorption-purification has been completedduring NOx regeneration or during SOx regeneration (sulfur poisoningrelease).

At step S4, a SOx accumulation quantity (simply, S quantity) of sulfuroxides accumulated into NOx trap catalyst 13 owing to sulfur poisoningis calculated. For instance, the SOx quantity can be calculated orestimated based on the integrated value of engine speeds, in the samemanner as the NOx quantity estimation method as previously discussed.That is, SOx quantity can be calculated or estimated based on theintegrated value of engine speeds or the traveling distance of thevehicle. Assuming that the integrated value of engine speeds is used forSOx quantity estimation, the integrated value must be reset to “0” atthe point of time when sulfur poisoning release (SOx regeneration) hasbeen completed.

At step S5, a PM accumulation quantity (simply, PM quantity) ofparticulate matter trapped and accumulated by DPF 14 is calculated. Theexhaust pressure of the outlet of DPF 14 tends to rise, as the PMquantity increases. Thus, it is possible to estimate the PM quantity bycomparing the exhaust pressure of the DPF inlet side, detected byexhaust pressure sensor 26, to a reference exhaust pressure, retrievedor determined based on the current engine operating conditions, such asthe current engine speed and the current engine load. In lieu thereof,the PM quantity may be estimated based on the combined informationaldata of the exhaust pressure detected by exhaust pressure sensor 26 andthe integrated value of engine speeds calculated from the time point ofcompletion of the previous DPF regeneration or the combinedinformational data of the exhaust pressure detected by exhaust pressuresensor 26 and the traveling distance calculated from the time point ofcompletion of the previous DPF regeneration.

At step S6, a check is made to determine whether a DPF regenerationperiod indicative flag regFLAG is set (=1). When the answer to step S6is in the affirmative (regFLAG=1), the routine proceeds from step S6 ofFIG. 2 to step S101 of the DPF regeneration control routine shown inFIG. 3. Conversely when the answer to step S6 is in the negative(regFLAG=0), the routine proceeds from step S6 to step S7.

At step S7, a check is made to determine whether a sulfur-poisoningrelease period indicative flag desulFLAG is set (=1). The flag state ofdesulFLAG=1 indicates that the system is executing the sulfur poisoningrelease mode for NOx trap catalyst 13. When the answer to step S7 isaffirmative (desulFLAG=1), the routine proceeds from step S7 to stepS201 of the sulfur poisoning release control routine shown in FIG. 4.Conversely when the answer to S7 is negative (desulFLAG=0), the routineproceeds from step S7 to step S8.

At step S8, a rich spike period indicative flag spFLAG is set (=1). Theflag state of spFLAG=1 indicates that the system is executing the richspike operating mode for NOx desorption-purification of NOx trapcatalyst 13. When the answer to step S8 is affirmative (spFLAG=1), theroutine proceeds from step S8 to step S301 of the rich spike controlroutine shown in FIG. 5. Conversely when the answer to S8 is negative(spFLAG=0), the routine proceeds from step S8 to step S9.

At step S9, a check is made to determine whether a melting lossprevention operating mode indicative flag recFLAG is set (=1). The flagstate of recFLAG=1 indicates that the system is executing the meltingloss prevention operating mode after the DPF regeneration and/or afterthe sulfur poisoning release, so as to prevent the DPF from being meltdown or damaged owing to an excessive rise of the DPF temperature. Whenthe answer to step S9 is affirmative (recFLAG=1), the routine proceedsfrom step S9 to step S401 of the melting loss prevention control routineshown in FIG. 6. Conversely when the answer to S9 is negative(recFLAG=0), the routine proceeds from step S9 to step S10.

At step S10, a check is made to determine whether a DPF-regenerationrequest indicative flag rq-DPFFLAG is set (=1). The flag state ofrq-DPFFLAG=1 indicates that there is a request for DPF regeneration.When the answer to step S10 is affirmative (rq-DPFFLAG=1), the routineproceeds from step S10 to step S501 of the order-of-priority decisionroutine shown in FIG. 7 for determining whether priority should be givento which of (i) DPF regeneration, (ii) NOx regeneration (rich spikemode), and (iii) SOx regeneration (sulfur poisoning release mode), inpresence of the request for DPF regeneration. Conversely when the answerto S10 is negative (rq-DPFFLAG=0), the routine proceeds from step S10 tostep S11.

At step S11, a check is made to determine whether a sulfur poisoningrelease request indicative flag rq-desulFLAG is set (=1). The flag stateof rq-desulFLAG=1 indicates that there is a request for sulfur poisoningrelease. When the answer to step S11 is affirmative (rq-desulFLAG=1),the routine proceeds from step S11 to step S601 of the order-of-prioritydecision routine shown in FIG. 8 for determining whether priority shouldbe given to which of (i) SOx regeneration (sulfur poisoning releasemode), (ii) NOx regeneration (rich spike mode), and (iii) DPFregeneration, in presence of the request for sulfur poisoning release.Conversely when the answer to S11 is negative (rq-desulFLAG=0), theroutine proceeds from step S11 to step S12.

At step S12, a check is made to determine whether the PM quantity,calculated through step S5, is greater than a predetermined PM quantityPM1. That is, the processor of control unit 20 determines, based on thecomparison result of the current PM quantity and predetermined PMquantity PM1, whether now is the best time for DPF regeneration. Whenthe answer to step S12 is affirmative (the current PM quantity>PM1), theroutine flows from step S12 to step S701 of the DPF-regeneration requestindicative flag rq-DPFFLAG setting routine shown in FIG. 9, so as to setDPF-regeneration request indicative flag rq-DPFFLAG to “1”. Converselywhen the answer to step S12 is negative (the current PM quantity≦PM1),the routine proceeds from step S12 to step S13.

At step S13, a check is made to determine whether the SOx quantity (Squantity), calculated through step S4, is greater than a predeterminedSOx quantity Si. That is, the processor of control unit 20 determines,based on the comparison result of the current SOx quantity andpredetermined SOx quantity S1, whether now is the best time for SOxregeneration (sulfur poisoning release). When the answer to step S13 isaffirmative (the current SOx quantity>S1), the routine flows from stepS13 to step S801 of the sulfur poisoning release request indicative flagrq-desulFLAG setting routine shown in FIG. 10, so as to set sulfurpoisoning release request indicative flag rq-desulFLAG to “1”.Conversely when the answer to step S13 is negative (the current SOxquantity≦S1), the routine proceeds from step S13 to step S14.

At step S14, a check is made to determine whether the NOx quantity,calculated through step S3, is greater than a predetermined NOx quantityNOx1. That is, the processor of control unit 20 determines, based on thecomparison result of the current NOx quantity and predetermined NOxquantity NOx1, whether now is the best time for NOx regeneration (NOxdesorption-purification). When the answer to step S14 is affirmative(the current NOx quantity>NOx1), the routine flows from step S14 to stepS901 of the rich spike request indicative flag rq-spFLAG setting routineshown in FIG. 11, so as to set rich spike request indicative flagrq-spFLAG to “1”. Conversely when the answer to step S14 is negative(the current NOx quantity≦NOx1), the routine of FIG. 2 returns to themain program.

Referring now to FIG. 3, there is shown the DPF regeneration controlroutine. The DPF regeneration control routine of FIG. 3 is initiatedunder a condition that the PM quantity reaches predetermined PM quantityPM1, DPF-regeneration request indicative flag rq-DPFFLAG is set (=1)through step S701 of FIG. 9, and thereafter DPF regeneration periodindicative flag regFLAG is set (=1) as a result of execution of theorder-of-priority decision routine (described later) of FIG. 7.

At step S101, for the purpose of DPF regeneration, the combustion modeof engine 1 is switched from a normal lean combustion mode to a splitretard combustion mode (hereunder described in detail).

The split retard combustion mode executed by the system of theembodiment is advantageously effectively used for various exhaustemission control purposes, such as DPF regeneration, sulfur poisoningrelease, NOx desorption-purification (rich spike), and warm-up promotion(rapid catalyst activation).

During execution of the DPF regeneration mode, the exhaust λ (the Greekletter λ denotes an excess air factor) must be controlled within aspecified range from 1 to 1.4, and additionally the DPF temperature mustbe kept above 600° C. On the other hand, during execution of the sulfurpoisoning release mode (SOx regeneration mode), the exhaust λ must becontrolled to be less than or equal to 1, and additionally the exhausttemperature must be kept to be greater than or equal to 600° C. When theengine is working in a normal operating range for lean combustionconditions, usually pilot injection is made. The pilot injection timing,exactly, the start of pilot injection (SOPI) is set to 40-100 ofcrank-angle (CA) before top dead center (BTDC) on the compressionstroke. The pilot injection quantity is set within an injection-quantityrange of 1 to 3 mm³/st. The main injection timing, exactly, the start ofmain injection (SOMI) is set to be within an injection-timing range from10° crankangle to −20° crankangle before TDC on the compression stroke.The time interval (the fuel-injector pulse width or time duration)between the end of pilot injection (EOPI) and the start of maininjection (SOMI) is usually set to be within a crankangle range from 10°to 30°.

In the normal engine operating range, in other words, during the normallean combustion operating mode, in order to achieve low-λ and highexhaust temperature conditions for the purpose of DPF regenerationand/or sulfur poisoning release, the quantity of intake air must bethrottled properly. However, throttling the intake-air quantity maycause a drop in an in-cylinder compression end temperature,corresponding to an in-cylinder temperature near TDC (the end of thecompression stroke), thus resulting in unstable combustion. Under such acondition, assuming that normal setting of pilot injection (that is,normal pilot injection timing and normal pilot injection quantity),suitable for the normal lean combustion operating mode, is used, it isnecessary to advance an injection timing of main injection, occurringafter the pilot injection (see the phase-advanced pulse indicating themain injection of FIG. 13A showing the first comparative example {circleover (1)}). For an exhaust temperature rise, it is desirable to retardthe main injection timing. However, in case of the previously-noted fuelinjection pattern as shown in FIG. 13A, exactly, the previously-notedsetting of the pilot injection timing, pilot injection quantity (thepulse width of pilot injection), main injection timing, and maininjection quantity (the pulse width of main injection), a phase-retardedamount of the main injection timing is considerably limited owing to theoccurrence of unstable combustion. Thus, it is difficult to realize thelow-λ and high exhaust temperature conditions for DPF regenerationand/or sulfur poisoning release, by way of the normal split fuelinjection pattern shown in FIGS. 13A-13B. In addition to the above, incase of the normal split injection pattern of the first comparativeexample {circle over (1)}, as can be seen from the heat release rateshown in FIG. 13B, combustion of the early-injection fuel portion bypilot injection and combustion of the late-injection fuel portion bymain injection tend to continuously occur. Thus, combustion of thelate-injection fuel portion may be mainly composed of diffusioncombustion rather than pre-mixed combustion.

In order to widen the phase-retardation limit of the main injectiontiming and to attain the low-λ and high exhaust temperature conditionsfor DPF regeneration and/or sulfur poisoning release, in the fuelinjection control system as disclosed in JP2000-320386, main injection,occurring after pilot injection, is further split into two pulses,namely the first main injection and the second main injection occurringafter the first main injection (see the two-split main injection of FIG.14A showing the second comparative example {circle over (2)}). However,in case of the conventional three-split injection pattern of the secondcomparative example {circle over (2)}, as can be seen from the heatrelease rate shown in FIG. 14B, the next fuel injection is initiatedduring a time period when combustion of the earlier-injection fuel islively developing. As a result, combustion of the early-injection fuelportion by pilot injection, combustion of the intermediate-injectionfuel portion by the first main injection, and combustion of thelate-injection fuel portion by the second main injection tend tocontinuously occur (see the sole ridged waveform of FIG. 14B indicatingthe heat release rate). That is, the intermediate-injection fuel portionby the first main injection is sprayed into the flame of theearly-injection fuel portion by pilot injection, and then thelate-injection fuel portion by the second main injection is sprayed intothe flame of the early-injection fuel portion and/or the flame of theintermediate-injection fuel portion. Under these conditions, as soon asthe first main injection initiates, the intermediate-injection fuelportion by the first main injection begins to burn at once. In a similarmanner, as soon as the second main injection initiates, thelate-injection fuel portion by the second main injection begins to burnat once. This causes an increased ratio of diffusion combustion topre-mixed combustion, and therefore an equivalent ratio, defined by aratio of a theoretical requirement to a quantity of air supplied,becomes rich partially in the combustion chamber. Such a rich equivalentratio produces increased exhaust emissions of smoke and particulates.

Taking the results of analyses of the split fuel injection patternsshown in FIGS. 13A-13B and 14A-14B into consideration, as can beappreciated from the fuel injection pattern and heat release rate shownin FIGS. 15A-15B, the combustion control system of the embodimentachieves a so-called split retard combustion mode {circle over (3)} bycontrolling fuel injection in a manner such that the system executes atleast two split fuel injection operation (see the preliminary fuelinjection denoted by “a” and the main fuel injection denoted by “b” inFIG. 15A), so that (i) a main combustion needed to produce a main enginetorque and (ii) at least one preliminary combustion occurring prior tothe main combustion are both achieved, and additionally the at least onepreliminary combustion takes place near TDC on the compression strokeand additionally the main combustion initiates after the preliminarycombustion has been completed. That is, as can be seen from theinjection pattern of FIG. 15A, fuel is first sprayed or injected on thecompression stroke of one complete operating cycle of the four-cycleengine 1 (see the preliminary injection “a” in FIG. 15A), for achievingthe preliminary combustion, required for a rise in in-cylindercompression end temperature near TDC (near the end of the compressionstroke). The fuel injection quantity of preliminary injection “a”,needed to create the heat release (see the comparatively small ridgedwaveform of FIG. 15B indicating the preliminary-combustion heat releaserate) at the preliminary combustion operation, is different depending onengine operating conditions. In the system of the embodiment, thepreliminary injection quantity of preliminary injection “a” is set to afuel injection quantity (see the characteristic map of FIG. 19) neededto ensure a heat release for preliminary combustion and additionallyneeded in order for an in-cylinder temperature obtained during the mainfuel injection period for main combustion to exceed a self-ignitabletemperature value. Also, it is possible to enhance the combustionstability of preliminary combustion by appropriately changing thepreliminary injection quantity and the preliminary injection timing ofpreliminary injection “a” needed for preliminary combustion, dependingon the compression end temperature, which is predicted based on each ofengine operating conditions. Please note that main injection “b” neededfor main combustion is initiated after TDC, so that the main combustion(see the comparatively large ridged waveform of FIG. 15B indicating themain-combustion heat release rate) initiates after the preliminarycombustion, occurring near TDC on the compression stroke owing topreliminary injection “a”, has been completed. According to the splitretard combustion mode {circle over (3)} of the system of theembodiment, it is possible to widen the retardation limit of the maincombustion by appropriately rising the in-cylinder temperature by way ofthe preliminary combustion, and thus to enhance the controllability tothe desired in-cylinder temperature (see the comparatively widened timeinterval (tm1−tp1) between the start tp1 of preliminary injection “a”and the start tm1 of main injection “b” in FIG. 15A and see thecomparatively widened time interval (tm2−tp2) between the start tp2 ofpreliminary combustion and the start tm2 of main combustion in FIG.15B). Additionally, it is possible to ensure the ignition delay duration(see the time duration Δt=(tm2−tm1) between the start tmi of maininjection “b” and the start tm2 of main combustion (the start of heatrelease during the main combustion operation) by initiating maininjection “b” after the preliminary combustion has been certainlycompleted (see the starting point tm1 of main injection “b” occurringafter the end tp3 of preliminary combustion in FIGS. 15A-15B). Theincreased ignition time duration Δt ensures an increased ratio ofpre-mixed combustion to diffusion combustion during the main combustionperiod, thereby resulting in reduced smoke emissions or reduced unburnedHC emissions. The time period (tm2−tp2) between the start tp2 ofpreliminary combustion (the start of heat release during the preliminarycombustion operation) and the start tm2 of main combustion variesdepending on engine speed Ne. It is desirable to retard the start tm2 ofmain combustion from the start tp2 of preliminary combustion by at least20 degrees of crankangle (20° CA), for initiating main combustion aftercompletion of preliminary combustion, in other words, after the heatrelease produced owing to preliminary combustion has completelyterminated (see the heat-release termination point tp3 of FIG. 15B). Byvirtue of setting of the time period (tm2−tp2) between the start tp2 ofpreliminary combustion and the start tm2 of main combustion at 20degrees or more of crankangle, it is possible to effectively suppress orprevent the main combustion from being deteriorated, thus suppressing orpreventing smoke emissions from increasing. In addition to the above,during the split retard combustion mode, the main combustion begins todevelop on the expansion stroke, and thus the combustion velocity isvery slow. As a result, the end tm3 of main combustion becomes at least50° CA or more after TDC on the compression stroke. In this manner, itis possible to realize the moderate main combustion operating mode byretarding the end tm3 of main combustion as much as possible, therebyeffectively reducing combustion noise.

FIG. 16 shows the comparison results of exhaust emission conditions,among the first comparative example {circle over (1)}, the secondcomparative example {circle over (2)}, and the split retard combustionmode {circle over (3)} of the system of the embodiment. As can be seenfrom the bar graphs of FIG. 16, even under a condition where the A/Fratio is set to rich (λ<1), it is possible to realize the improvedcombustion operating mode that ensures high exhaust temperatures and lowsmoke emissions. Additionally, as can be seen from the rightmost bargraph of FIG. 16, during the split retard combustion mode, it ispossible to realize relatively low HC emissions.

Furthermore, as previously discussed, the system of the embodimentwidens the retardation limit of main combustion by appropriately risingthe in-cylinder temperature by way of preliminary combustion, and thusit is possible to ensure stable combustion under a low-λ condition andto realize high exhaust temperatures, even when the main injectiontiming, exactly, the start tm1 of main injection “b” is retarded.

Referring now to FIGS. 17A-17D, there are shown the characteristiccurves respectively indicating the exhaust temperature, smoke density,CO density, and HC density, during the main injection period. Asappreciated, the greater the main injection period retards (in otherwords, the main combustion timing retards), the greater the ratio ofpre-mixed combustion to diffusion combustion increases. Even under thelow-λ condition, it is possible to realize remarkable smoke emissionssuppression by retarding the main combustion timing as much as possible(see FIG. 17B). On the other hand, due to the retarded main combustiontiming, the CO emissions density tends to become somewhat high (see FIG.17C), whereas the HC emissions density can be held at the low level (seeFIG. 17D). Additionally, by way of retardation of the main combustiontiming, it is possible to realize higher exhaust temperatures (see FIG.17A). That is, it is possible to properly control the exhausttemperature by properly changing the injection timing of main injection“b” for main combustion.

Referring to FIG. 18, there is shown the predetermined preliminaryinjection timing characteristic map showing how a desired fuel injectiontiming for preliminary injection needed for preliminary combustion hasto be varied relative to engine operating conditions, namely enginespeed Ne and engine load Q.

Referring to FIG. 19, there is shown the predetermined preliminaryinjection quantity characteristic map showing how a desired fuelinjection quantity for preliminary combustion has to be varied relativeto engine operating conditions, namely engine speed Ne and engine loadQ.

Referring to FIG. 20, there is shown the predetermined main injectiontiming characteristic map showing how a desired fuel injection timingfor main injection needed for main combustion has to be varied relativeto engine operating conditions, namely engine speed Ne and engine loadQ, in order to realize a certain desired exhaust temperature. On theother hand, a desired fuel injection quantity for main combustion (inother words, a main injection quantity) is determined based on a desiredλ and a desired air quantity tQac, which is determined in accordancewith the air quantity control routine (described later) of FIG. 25containing torque compensation (torque increase) needed to compensatefor an engine torque drop occurring due to main-combustion retardationduring the split retard combustion mode.

In particular, during low engine load operation, in order to achieve thedesired exhaust temperature the heat release starting point tm2 of maincombustion tends to considerably retard, since the injection timing ofmain injection “b” is considerably retarded during low engine loadoperation (see the map of FIG. 20). Assuming that fuel is injected onlyonce as preliminary injection and thus only one preliminary combustionoccurs near TDC on the compression stroke under such a low engine loadcondition, there is a possibility that the in-cylinder temperaturebecomes less than the self-ignitable temperature value during the maininjection period for main combustion operation. In such a case, that is,during low loads, as can be seen from the fuel injection pattern andheat release rate shown in FIGS. 21A-21B, it is possible to maintain thein-cylinder temperature above the self-ignitable temperature value byperforming preliminary combustion two or more times, so that theheat-release time periods of two adjacent preliminary combustionoperations are not overlapped with each other (see the comparativelysmall two-split ridged waveforms of FIG. 21B). In such a multipreliminary combustion plus main combustion mode {circle over (4)} shownin FIGS. 21A-21B, it is preferable that at least one of the two or morepreliminary combustion operations occurs near TDC on the compressionstroke (see FIG. 21B). The multi preliminary combustion plus maincombustion mode {circle over (4)} shown in FIGS. 21A-21B (a combinationof at least two preliminary combustion operations and main combustionoperation split apart from each other) reconciles both of low smokeemissions and high exhaust temperatures, even under low engine loadoperation.

As discussed above, in presence of a request for low-λ and high exhausttemperature for DPF regeneration and/or sulfur poisoning release, thesystem of the embodiment executes switching from the normal combustionmode to the split retard combustion mode.

Concretely, mode switching to the split retard combustion mode isachieved in accordance with the flow chart shown in FIG. 22.

At step S1101 of FIG. 22, preliminary fuel injection needed forpreliminary combustion is initiated and executed with a desiredpreliminary injection quantity determined or retrieved from thepredetermined map shown in FIG. 19 at a desired preliminary injectiontiming determined or retrieved from the predetermined map shown in FIG.18.

At step S1102, main fuel injection needed for main combustion isinitiated and executed at a retarded injection timing (see the map ofFIG. 20) after TDC on the compression stroke, with a desired maininjection quantity increasingly compensated for torque compensationneeded to compensate for the engine torque fall occurring due to theremarkable main injection timing retard. Such torque compensation isneeded to minimize the engine torque difference before and afterswitching to the split retard combustion mode.

Returning to step S102 of FIG. 3, the exhaust λ is controlled to adesired value ranging from “1” to “1.4”. During the DPF regenerationperiod, the desired value of the exhaust λ varies depending on the PMquantity. Therefore, first, the PM quantity is estimated by comparingthe exhaust pressure of the DPF inlet side, detected by exhaust pressuresensor 26, to the reference exhaust pressure, determined based on thecurrent engine operating conditions, such as the current engine speedand the current engine load. Next, a desired λ is determined based onthe estimated PM quantity from the preprogrammed characteristic map ofFIG. 23 showing how the desired λ has to be varied relative to the PMquantity. Details of A/F control executed to attain the desired λ willbe described later in reference to the flow charts shown in FIG. 25 (airquantity control containing torque compensation) and FIG. 26 (fuelinjection quantity control). Briefly, during the split retard combustionmode, the A/F control to attain the desired λ is executed, whilesimultaneously executing torque compensation needed to compensate for atorque decrease occurring due to main-combustion retardation.

At step S103, a check is made to determine whether the DPF temperatureexceeds a predetermined DPF-regeneration period upper limit T22 (anupper limit of the desired DPF temperature during DPF regeneration).When the condition defined by DPF temperature>T22 is satisfied, theprocessor of control unit 20 determines that the DPF temperature exceedsupper limit T22 during the DPF regeneration mode, and thus the routineproceeds from step S103 to step S111, in order to advance the maininjection timing and thus to decreasingly compensate for the exhausttemperature.

At step S111, a temperature deviation ΔT (=DPF temperature−T22) betweenthe actual DPF temperature and upper limit T22 is calculated.

At step S112, a feedback gain G is calculated and determined based ontemperature deviation ΔT. For the purpose of a rapid convergence of theactual DPF temperature to the desired temperature value, feedback gain Gis set to increase, as temperature deviation ΔT increases. Converselywhen temperature deviation ΔT reduces, feedback gain G is decreasinglycompensated for, to prevent undesired overshoot.

At step S113, a phase-advanced amount (a feedback compensation amount)tadvance of main injection timing for main combustion is calculated bymultiplying temperature deviation ΔT by feedback gain G, from theexpression tadvance=ΔT×G. As can be appreciated from the expressiontadvance=ΔT×G, as feedback compensation for main injection timing, thesystem of the embodiment uses proportional control in which the amountof corrective action is proportional to the amount of error (ΔT=DPFtemperature−T22). In lieu thereof, the system of the embodiment mayutilize integral control, derivative control, proportional-plus-integralcontrol in which the control signal is a linear combination of the errorsignal and its integral, or proportional-plus-derivative control inwhich the control signal is a linear combination of the error signal andits derivative.

At step S114, the main injection timing is advanced by phase-advancedamount tadvance, calculated through step S113, so as to lower theexhaust temperature. Thus, even when an undesirably excessive DPFtemperature rise occurs, the system of the embodiment can rapidly dropthe exhaust temperature and consequently protect or prevent DPF 14 frombeing damaged.

Returning to step S103, if the condition defined by DPF temperature>T22is unsatisfied, that is, in case of DPF temperature≦T22, the routineproceeds from step S103 to step S104.

At step S104, a check is made to determine whether the DPF temperatureis less than a predetermined DPF-regeneration period lower limit T21 (alower limit of the desired DPF temperature during DPF regeneration).When the condition defined by DPF temperature<T21 is satisfied, theprocessor determines that the DPF temperature becomes less than lowerlimit T21 during the DPF regeneration mode, and thus the routineproceeds from step S104 to step S110.

At step S110, the main injection timing is retarded by a predeterminedamount, to increasingly compensate for the exhaust temperature.

Returning to step S104, if the condition defined by DPF temperature<T21is unsatisfied, that is, in case of DPF temperature≧T21, the routineproceeds from step S104 to step S105.

At step S105, a check is made to determine whether a predetermined timeperiod tdpfreg has expired from the time when the DPF regeneration modeis initiated. Expiration of predetermined time period tdpfreg means thatPM accumulated in DPF 14 has certainly burned and removed from DPF 14and thus DPF regeneration has been completed. Therefore, when the answerto step S105 is affirmative (YES), that is, upon expiration ofpredetermined time period tdpfreg, the routine proceeds from step S105to step S106.

At step S106, the combustion mode is changed from the split retardcombustion mode to the normal combustion mode, because of completion ofDPF regeneration. In this manner heating operation of DPF 14 terminates.

At step S107, owing to completion of DPF regeneration, DPF regenerationperiod indicative flag regFLAG is reset (=0) At step S108, melting lossprevention operating mode indicative flag recFLAG is set (=1), sincethere is a possibility that the unburned particulate matter isinstantaneously burned out owing to a rapid change (a rapid rise) in theexhaust λ in the event that the unburned PM still exists in DPF 14 aftercompletion of DPF regeneration, and such instantaneous combustion of PMresults in melting loss of DPF 14. After setting of flag recFLAG to “1”,the melting loss prevention operating mode (described later in referenceto the flow of FIG. 6) is initiated.

Referring now to FIG. 4, there is shown the sulfur poisoning releasecontrol routine. The sulfur poisoning release control routine of FIG. 4is initiated under a condition that the S quantity of SOx in NOx trapcatalyst 13 reaches predetermined S quantity S1, sulfur poisoningrelease request indicative flag rq-desulFLAG is set (=1) through stepS801 of FIG. 10, and thereafter sulfur-poisoning release periodindicative flag desulFLAG is set (=1) as a result of execution of theorder-of-priority decision routine (described later) of FIG. 8.

At step S201, for the purpose of sulfur poisoning release (SOxregeneration), the combustion mode of engine 1 is switched from thenormal lean combustion mode to the previously-discussed split retardcombustion mode.

At step S202, the exhaust λ is controlled to a desired value (λ=1)corresponding to a stoichiometric A/F ratio (14.7:1). Details of A/Fcontrol executed to attain the desired λ will be described later inreference to the flow charts shown in FIG. 25 (air quantity controlcontaining torque compensation) and FIG. 26 (fuel injection quantitycontrol). Briefly, during the split retard combustion mode, the A/Fcontrol to attain the desired λ is executed, while simultaneouslyexecuting torque compensation needed to compensate for a torque decreaseoccurring due to main-combustion retardation.

At step S203, a check is made to determine whether the catalysttemperature exceeds a predetermined SOx-regeneration period upper limitT42 (an upper limit of the desired catalyst temperature during SOxregeneration). When the condition defined by catalyst temperature>T42 issatisfied, the processor of control unit 20 determines that the catalysttemperature exceeds upper limit T42 during the SOx regeneration mode,and thus the routine proceeds from step S203 to step S211, in order toadvance the main injection timing and thus to decreasingly compensatefor the exhaust temperature.

At step S211, a temperature deviation ΔT (=catalyst temperature−T42)between the actual catalyst temperature and upper limit T42 iscalculated.

At step S212, a feedback gain G is calculated and determined based ontemperature deviation ΔT. For the purpose of a rapid convergence of theactual catalyst temperature to the desired temperature value, feedbackgain G is set to increase, as temperature deviation ΔT increases.Conversely when temperature deviation ΔT reduces, feedback gain G isdecreasingly compensated for, to prevent undesired overshoot.

At step S213, a phase-advanced amount (a feedback compensation amount)tadvance of main injection timing for main combustion is calculated bymultiplying temperature deviation ΔT by feedback gain G, from theexpression tadvance=ΔT×G. As can be appreciated from the expressiontadvance =ΔT×G, as feedback compensation for main injection timing, thesystem of the embodiment uses proportional control in which the amountof corrective action is proportional to the amount of error (ΔT=catalysttemperature−T42). In lieu thereof, the system of the embodiment mayutilize integral control, derivative control, proportional-plus-integralcontrol in which the control signal is a linear combination of the errorsignal and its integral, or proportional-plus-derivative control inwhich the control signal is a linear combination of the error signal andits derivative.

At step S214, the main injection timing is advanced by phase-advancedamount tadvance, calculated through step S213, so as to lower theexhaust temperature. Thus, even when an undesirably excessive catalysttemperature rise occurs, the system of the embodiment can rapidly dropthe exhaust temperature and consequently protect or prevent NOx trapcatalyst 13 from being damaged.

Returning to step S203, if the condition defined by catalysttemperature>T42 is unsatisfied, that is, in case of catalysttemperature≦T42, the routine proceeds from step S203 to step S204.

At step S204, a check is made to determine whether the catalysttemperature is less than a predetermined SOx-regeneration period lowerlimit T41 (a lower limit of the desired catalyst temperature during SOxregeneration). When the condition defined by catalyst temperature<T41 issatisfied, the processor determines that the catalyst temperaturebecomes less than lower limit T41 during the SOx regeneration mode, andthus the routine proceeds from step S204 to step S210.

At step S210, the main injection timing is retarded by a predeterminedamount, to increasingly compensate for the exhaust temperature. 15Returning to step S204, if the condition defined by catalysttemperature<T41 is unsatisfied, that is, in case of catalysttemperature≧T41, the routine proceeds from step S204 to step S205.

At step S205, a check is made to determine whether a 20 predeterminedtime period tdesul has expired from the time when the SOx regenerationmode is initiated. Expiration of predetermined time period tdesul meansthat SOx trapped by NOx trap catalyst 13 has certainly desorbedtherefrom and thus SOx regeneration (sulfur poisoning release) has been25 completed. Therefore, when the answer to step S205 is affirmative(YES), that is, upon expiration of predetermined time period tdesul, theroutine proceeds from step S205 to step S206.

At step S206, the combustion mode is changed from the 30 split retardcombustion mode to the normal combustion mode, because of completion ofSOx regeneration. In this manner heating operation of NOx trap catalyst13 terminates.

At step S207, owing to completion of SOx regeneration, sulfur-poisoningrelease period indicative flag desulFLAG is reset (=0).

At step S208, melting loss prevention operating mode indicative flagrecFLAG is set (=1), since there is a possibility that the unburnedparticulate matter is instantaneously burned out owing to a rapid change(a rapid rise) in the exhaust λ in the event that the unburned PM existsin DPF 14 under such high temperature conditions after completion of SOxregeneration, and such instantaneous combustion of PM results in meltingloss of DPF 14. After setting of flag recFLAG to “1”, the melting lossprevention operating mode (described later in reference to the flow ofFIG. 6) is initiated.

At step S209, rich spike request indicative flag rq-spFLAG is reset to“0”. This is because NOx trap catalyst 13 is subjected to thestoichiometric combustion for a long time period during the sulfurpoisoning release mode, and thus there is an increased tendency for theNOx desorption-purification mode (NOx regeneration mode or rich spikemode) to be simultaneously performed. If the rich spike mode (NOxregeneration mode) has already been required and thus rich spike requestindicative flag rq-spFLAG has already been set (=1), it is necessary toinhibit the rich spike mode. For this reason, resetting of rich spikerequest indicative flag rq-spFLAG is made at step S209.

Referring now to FIG. 5, there is shown the rich spike control routine(the NOx desorption-purification control routine or NOx regenerationcontrol routine). The rich spike control routine of FIG. 5 is initiatedunder a condition that the NOx quantity of NOx trap catalyst 13 reachespredetermined NOx quantity NOx1, rich spike request indicative flagrq-spFLAG is set (=1) through step S901 of FIG. 11, and thereafter richspike period indicative flag spFLAG is set (=1) as a result of executionof either the order-of-priority decision routine (described later) ofFIG. 7 or the order-of-priority decision routine (described later) ofFIG. 8.

At step S301, for the purpose of NOx regeneration, the combustion modeof engine 1 is switched from the normal lean combustion mode to thepreviously-discussed split retard combustion mode.

At step S302, the exhaust λ is controlled to a desired value (λ<1)corresponding to a rich A/F ratio. Details of A/F control executed toattain the desired λ (<1) will be described later in reference to theflow charts shown in FIG. 25 (air quantity control containing torquecompensation) and FIG. 26 (fuel injection quantity control). Briefly,during the split retard combustion mode, the A/F control to attain thedesired λ is executed, while simultaneously executing torquecompensation needed to compensate for a torque decrease occurring due tomain-combustion retardation.

At step S303, a check is made to determine whether a predetermined timeperiod tspike has expired from the time when the NOx regeneration modeis initiated. Expiration of predetermined time period tspike means thatNOx trapped by NOx trap catalyst 13 has certainly desorbed therefrom andthus NOx regeneration (NOx desorption-purification) has been completed.Therefore, when the answer to step S303 is affirmative (YES), that is,upon expiration of predetermined time period tspike, the routineproceeds from step S303 to step S304.

At step S304, the combustion mode is changed from the split retardcombustion mode to the normal combustion mode, because of completion ofNOx regeneration. As a matter of course, at the same time, the richspike mode, creating a transiently considerably richened A/F ratio(λ<1), terminates.

At step S305, owing to completion of NOx regeneration, rich spike periodindicative flag spFLAG is reset (=0).

Referring now to FIG. 6, there is shown the DPF melting loss preventioncontrol routine. The DPF melting loss prevention control routine of FIG.6 is initiated under a condition that melting loss prevention operatingmode indicative flag recFLAG is set (=1) after completion of DPFregeneration as a result of execution of the routine of FIG. 3 ormelting loss prevention operating mode indicative flag recFLAG is set(=1) after completion of SOx regeneration (sulfur poisoning release) asa result of execution of the routine of FIG. 4. As previously described,there is a possibility that unburned PM existing in DPF 14 isinstantaneously burned out if the exhaust λ is rapidly changed to lean(λ>1) under high temperature conditions just after completion of DPFregeneration, and thus such instantaneous combustion of PM results inmelting loss of DPF 14.

Thus, at step S401, the exhaust λ is controlled to the desired value(λ≦1.4). During the DPF melting loss prevention mode, it is desirable tocontrol or keep the exhaust temperature at comparatively low temperaturevalues. Therefore, the exhaust λ is controlled to the desired value byway of the normal combustion mode instead of using the split retardcombustion mode.

At step S402, a check is made to determine whether the DPF temperaturebecomes less than a predetermined temperature value T3, for example 500°C., that there is no risk of initiating rapid oxidation of PMaccumulated in DPF 14. When the answer to step S402 is negative (DPFtemperature≧T3), the system repeatedly continuously executes the exhaustλ control (λ≦1.4). Conversely when the answer to step S402 isaffirmative (DPF temperature<T3), the processor of control unit 20determines that it is possible to avoid melting loss (melt-down) of DPF14 even when the concentration of oxygen, exactly, the percentage ofoxygen contained within the engine exhaust gases, becomes equal to thatof the atmosphere. Thus, in case of DPF temperature<T3, the routineproceeds from step S402 to step S403.

At step S403, the exhaust λ control (λ≦1.4) terminates, since there is aless possibility of DPF melting loss (DPF melt-down) under the conditionof DPF temperature<T3.

At step S404, simultaneously with termination of the DPF melting lossprevention mode, melting loss prevention operating mode indicative flagrecFLAG is reset (=0).

Referring now to FIG. 7, there is shown the order-of-priority decisionroutine executed in presence of the request for DPF regeneration (withDPF-regeneration request indicative flag rq-DPFFLAG set (=1)).Concretely, when DPF regeneration is required (see the flow from stepS601 of FIG. 8 through step S701 of FIG. 9 to step S10 of FIG. 2), theorder-of-priority decision routine of FIG. 7 is initiated. Theorder-of-priority decision routine of FIG. 7 is executed to determinewhether priority should be given to which of (i) DPF regeneration, (ii)NOx regeneration (rich spike mode), and (iii) SOx regeneration (sulfurpoisoning release mode), when the request for SOx regeneration (sulfurpoisoning release mode) and the request for NOx regeneration (rich spikemode) occur simultaneously, in presence of the request for DPFregeneration.

At step S501, in a similar manner to step S13 of FIG. 2, a check is madeto determine whether the SOx quantity reaches predetermined SOx quantityS1 after the request for DPF regeneration has occurred and the flagrq-DPFFLAG has set. When the answer to step S501 is affirmative (thecurrent SOx quantity>S1), the routine flows from step S501 to step S801of FIG. 10, so as to set sulfur poisoning release request indicativeflag rq-desulFLAG to “1”. Conversely when the answer to step S501 isnegative (the current SOx quantity≦S1), the routine proceeds from stepS501 to step S502.

At step S502, a check is made to determine whether rich spike requestindicative flag rq-spFLAG is set (=1). When the request for NOxdesorption-purification (rich spike mode) is absent (rq-spFLAG=0), theroutine proceeds from step S502 to step S503.

At step S503, in a similar manner to step S14 of FIG. 2, a check is madeto determine whether the NOx quantity reaches predetermined NOx quantityNOx1 after the request for DPF regeneration has occurred and thus theflag rq-DPFFLAG has set. When the answer to step S503 is affirmative(the current NOx quantity>NOx1), the routine flows from step S503 tostep S901 of FIG. 11, so as to set rich spike request indicative flagrq-spFLAG to “1”. Conversely when the answer to step S503 is negative(the current NOx quantity≦NOx1), the routine proceeds from step S503 tostep S504.

At step S504, a check is made to determine whether the current engineoperating range is within the preprogrammed DPF-regeneration andsulfur-poisoning-release enabling region (DPF, SOx regeneration enablingregion). As can be seen from the predetermined Ne-Q versus DPF, SOxregeneration enabling region characteristic diagram of FIG. 24, the DPF,SOx regeneration enabling region is set outside of low-speed andlow-load operating range in which a margin for an exhaust temperaturerise is relatively small and a margin for deterioration in exhaustperformance does not exceed an allowable level. In other words, the DPF,SOx regeneration disabling region is set within the low-speed andlow-load operating range. When the answer to step S504 is affirmative(YES), and thus the current engine operating range is within thepreprogrammed DPF, SOx regeneration enabling region, the routineproceeds from step S504 to step S505.

At step S505, DPF regeneration period indicative flag regFLAG is set(=1). As a result of this, the DPF regeneration mode is preferentiallyexecuted.

Returning to step S502, if the request for NOx desorption-purification(rich spike mode) is present (rq-spFLAG=1), the processor of controlunit 20 determines that the request for DPF regeneration and the requestfor NOx regeneration are both present, and thus the routine proceedsfrom step S502 to step S506.

At step S506, a check is made to determine whether the current engineoperating condition is conditioned in a low-NOx-emissions condition,such as a steady-state operating condition. When the answer to step S506is affirmative (YES), the processor determines that there is a lessdeterioration in exhaust emissions of the exhaust tailpipe even when theNOx regeneration mode for NOx trap catalyst 13 is initiated with a timelag. Thus, the processor decides that priority should be given to DPFregeneration by which the drivability may be greatly affected. For thereasons discussed above, under low-NOx emissions condition, the routineflows from step S506 to step S507. Conversely when the answer to stepS506 is negative (NO), the processor determines that it is necessary toprevent a remarkable deterioration in exhaust emissions of the exhausttailpipe under high-NOx emissions condition. Thus, the processor decidesthat priority should be given to NOx regeneration. Thus, under high-NOxemissions condition, the routine flows from step S506 to step S508.

At step S508, rich spike period indicative flag spFLAG is set (=1), andthus the NOx regeneration mode is preferentially executed.

At step S507, a check is made to determine whether the DPF temperatureexceeds a predetermined oxidation catalyst activation temperature valueT6 that is required for activating the oxidation catalyst provided inDPF 14. As may be appreciated, when starting to rise the exhausttemperature under the condition of the DPF temperature belowpredetermined temperature value T6, it takes a long time until the DPFregeneration enabling temperature is reached. This may deteriorateexhaust emissions of the exhaust tailpipe. Thus, under the condition ofthe DPF temperature below predetermined temperature value T6, priorityshould be given to NOx regeneration. Thus, when DPF temperature≦T6, theroutine also flows from step S507 to step S508. At step S508, rich spikeperiod indicative flag spFLAG is set (=1), and thus the NOx regenerationmode is preferentially executed.

Conversely when the answer to step S507 is affirmative (YES), that is,when DPF temperature>T6, the routine flows from step S507 via step S504to step S505, so as to set DPF regeneration period indicative flagregFLAG, and consequently to initiate the DPF regeneration modepreferentially.

Referring now to FIG. 8, there is shown the order-of-priority decisionroutine executed in presence of the request for SOx regeneration (withsulfur poisoning release request indicative flag rq-desulFLAG set (=1)).Concretely, when SOx regeneration is required (see the flow from stepS501 of FIG. 7 through step S801 of FIG. 10 to step S11 of FIG. 2), theorder-of-priority decision routine of FIG. 8 is initiated. Theorder-of-priority decision routine of FIG. 8 is executed to determinewhether priority should be given to which of (i) SOx regeneration(sulfur poisoning release mode), (ii) NOx regeneration (rich spikemode), and (iii) DPF regeneration, when the request for NOx regeneration(rich spike mode) and the request for DPF regeneration occursimultaneously, in presence of the request for SOx regeneration (sulfurpoisoning release mode).

At step S601, in a similar manner to step S12 of FIG. 2, a check is madeto determine whether the PM quantity reaches predetermined PM quantityPM1 after the request for sulfur poisoning release has occurred and theflag rq-desulFLAG has set. When the answer to step S601 is affirmative(the current PM quantity>PM1), the routine flows from step S601 to stepS701 of FIG. 9, so as to set DPF-regeneration request indicative flagrq-DPFFLAG to “1”. Conversely when the answer to step S601 is negative(the current PM quantity≦PM1), the routine proceeds from step S601 tostep S602.

At step S602, a check is made to determine whether the catalysttemperature exceeds a predetermined temperature value T1. As may beappreciated, when starting to rise the exhaust temperature under thecondition of the catalyst temperature below predetermined temperaturevalue T1, it takes a long time until the sulfur-poisoning-releaseenabling temperature is reached. This may deteriorate exhaust emissionsof the exhaust tailpipe. Thus, under the condition of the catalysttemperature below predetermined temperature value Ti, priority should begiven to NOx regeneration. Thus, when catalyst temperature≦T1, theroutine proceeds from step S602 to step S605. Conversely when the answerto step S602 is affirmative (YES), that is, when catalysttemperature>T1, the routine flows from step S602 to step S603.

At step S603, a check is made to determine whether the current engineoperating range is within the preprogrammed DPF-regeneration andsulfur-poisoning-release enabling region (DPF, SOx regeneration enablingregion). When the answer to step S603 is affirmative (YES), and thus thecurrent engine operating range is within the preprogrammed DPF, SOxregeneration enabling region, the routine proceeds from step S603 tostep S604.

At step S604, sulfur poisoning release request indicative flagrq-desulFLAG is set (=1). As a result of this, the sulfur poisoningrelease mode (SOx regeneration) is preferentially executed.

At step S605, a check is made to determine whether rich spike requestindicative flag rq-spFLAG is set (=1). When the request for NOxdesorption-purification (rich spike mode) is present (rq-spFLAG=1), theroutine proceeds from step S605 to step S607.

At step S607, rich spike period indicative flag spFLAG is set (=1), andthus the NOx regeneration mode is preferentially executed.

Returning to step S605, if the request for NOx desorption-purification(rich spike mode) is absent (rq-spFLAG=0), the routine flows from stepS605 to step S606.

At step S606, in a similar manner to step S14 of FIG. 2 or step S503 ofFIG. 7, a check is made to determine whether the NOx quantity reachespredetermined NOx quantity NOx1 after the request for sulfur poisoningrelease has occurred and thus the flag rq-desulFLAG has set. When theanswer to step S606 is affirmative (the current NOx quantity>NOx1), theroutine flows from step S606 to step S901 of FIG. 11, so as to set richspike request indicative flag rq-spFLAG to “1”.

Referring now to FIG. 12, there is shown the warm-up promotion controlroutine. Concretely, when the catalyst temperature is less than or equalto a predetermined catalyst activation temperature value T5, the warm-uppromotion control routine of FIG. 12 is initiated for rapid activationof the NOx trap catalyst.

At step S1001, a check is made to determine whether the current engineoperating range is within a preprogrammed warm-up promotionengine-operation enabling region (simply, warm-up promotion enablingregion). In the system of the embodiment, the warm-up promotingoperation of engine 1 is achieved by way of the split retard combustionmode (see FIGS. 15A-15B, and 21A-21B). Therefore, the warm-up promotionenabling region can be regarded as to be almost equivalent to the splitretard combustion mode enabling engine-operating range. As previouslydescribed, the system of the embodiment utilizes the split retardcombustion mode, for DPF regeneration, sulfur poisoning release, or NOxregeneration. That is, the warm-up promotion enabling region can beregarded as to be almost equivalent to the previously-discussedpreprogrammed DPF-regeneration and sulfur-poisoning-release enablingregion. For the reasons set out above, in the system of the shownembodiment, the predetermined warm-up promotion enabling region is setto be identical to the preprogrammed DPF, SOx regeneration enablingregion shown in FIG. 24. When the answer to step S1001 is affirmative(YES), and thus the current engine operating range is within thepredetermined warm-up promotion enabling region (the preprogrammed DPF,SOx regeneration enabling region of FIG. 24), the routine proceeds fromstep S1001 to step S1002.

At step S1002, for warm-up promotion, the combustion mode of engine 1 isswitched from the normal lean combustion mode to the split retardcombustion mode. After switching to the split retard combustion mode, itis possible to rapidly rise the exhaust temperature. This effectivelypromotes a rapid catalyst warm-up (a rapid catalyst activation). Duringthe warm-up promotion mode, the exhaust λ is controlled to the desiredvalue. Details of A/F control executed to attain the desired λ will bedescribed later in reference to the flow charts shown in FIG. 25 (airquantity control containing torque compensation) and FIG. 26 (fuelinjection quantity control). Briefly, during the split retard combustionmode for catalyst warm-up purposes, the A/F control to attain thedesired λ is executed, while simultaneously executing torquecompensation needed to compensate for a torque decrease occurring due tomain-combustion retardation.

At step S1003, a check is made to determine whether the catalysttemperature of NOx trap catalyst 13 is greater than predeterminedcatalyst activation temperature value T5. When the answer to step S1003is affirmative (catalyst temperature>T5), the routine proceeds from stepS1003 to step S1004.

At step S1004, the combustion mode is changed from the split retardcombustion mode to the normal combustion mode, because of completion ofcatalyst warm-up promotion. In this manner, heating operation of NOxtrap catalyst 13 terminates.

Next, details of A/F control executed to attain the desired λ aredescribed hereunder in reference to the flow charts shown in FIG. 25(air quantity control containing torque compensation) and FIG. 26 (fuelinjection quantity control). During the split retard combustion mode,the engine torque tends to decrease, and therefore it is important toexecute optimum torque compensation for minimizing the engine torquedifference before and after switching to the split retard combustionmode, while keeping the desired λ. The higher the rate of exhausttemperature rise, caused by retardation of the main injection timing formain combustion (see FIG. 17A), the greater the engine torque decreasebecomes. The countermeasure against such an engine torque drop occurringowing to the main-injection-timing retard is necessary. In particular,during the warm-up period, even when the main fuel injection timing formain combustion is the same, there is an increased tendency that locallylow-temperature portions exist within the combustion chamber. Suchlocally low-temperature portions cause the undesirably loweredcombustion efficiency, thus resulting in the remarkable engine torquedrop. The countermeasure against such a problem is also necessary.

Referring now to FIG. 25, there is shown the air quantity controlroutine containing engine torque compensation. The air quantity controlroutine of FIG. 25 is also executed as time-triggered interrupt routinesto be triggered every predetermined time intervals.

At step S2001, a required fuel injection quantity QFDRV, correspondingto a required engine torque, is calculated or determined based onaccelerator opening APO and engine speed Ne.

At step S2002, a basic target air quantity tQacb is calculated ordetermined based on the required fuel injection quantity QFDRV and thedesired λ. As will be appreciated from the above, the value of thedesired λ is different depending on which of (i) DPF regeneration, (ii)NOx regeneration (rich spike mode), (iii) SOx regeneration (sulfurpoisoning release mode), and (iv) catalyst warm-up promotion isselectively executed during the split retard combustion period. Thedesired λ used for the split retard combustion mode is determined to beadequately relatively smaller rather than a desired λ used for thenormal lean combustion mode. Thus, basic target air quantity tQacbsuitable for the split retard combustion period can be set to beadequately relatively smaller than that of the normal lean combustionmode.

At step S2003, a check is made to determine whether the engine isoperated in the split retard combustion mode. When the answer to stepS2003 is affirmative (during the split retard combustion mode), theroutine proceeds from step S2003 to step S2004. Conversely when theanswer to step S2003 is negative (out of the split retard combustionmode), the routine proceeds from step S2003 to step S2009.

At step S2004, a basic torque correction factor Ka1 is calculated ordetermined based on engine speed Ne and the desired λ, from thepreprogrammed Ne-λ-Ka1 characteristic map shown in FIG. 27. The smallerthe desired λ from the value “1” corresponding to the stoichiometric A/Fratio, the greater the engine torque drop. Basic torque correctionfactor Ka1 is increasingly compensated for, as the desired A varies fromstoichiometric (λ=1) to rich (λ<1) (see FIG. 27). Additionally, for thesame burn time (or the same combustion time), the engine torque tends tofall due to a crank-angle change, as engine speed Ne increases. Thus,basic torque correction factor Ka1 is also increasingly compensated for,as engine speed Ne increases (see FIG. 27).

At step S2005, a torque correction factor Ka2 is calculated ordetermined based on the main injection timing of main injection “b” (seeFIG. 15A). Concretely, torque correction factor Ka2 is retrieved basedon the main injection timing from the preprogrammed characteristic mapshown in FIG. 28. As seen from the map of FIG. 28, the greater the maininjection timing becomes retarded, the greater torque correction factorKa2 becomes. This is because the engine torque drop tends to increase,as the retardation amount of the main injection timing increases.

At step S2006, a water-temperature dependent torque correction factorKa3 is calculated or determined based on the engine temperature (coolanttemperature Tw). Concretely, torque correction factor Ka3 is retrievedbased on the engine temperature Tw, from the preprogrammed Tw−Ka3characteristic map shown in FIG. 29. As seen from the map of FIG. 29,the lower the engine temperature Tw, the greater torque correctionfactor Ka3 becomes. This is because the combustion efficiency tends todeteriorate and thus engine torque tends to drop, as the enginetemperature Tw becomes lower. Such torque compensation based onwater-temperature dependent torque correction factor Ka3 is veryeffective during the catalyst warm-up promotion mode.

At step S2007, a final torque correction factor Ka is calculated basedon all of correction factors Ka1, Ka2, and Ka3, from the expressionKa=Ka1×Ka2×Ka3.

At step S2008, a target air quantity tQac is calculated or determinedbased on final correction factor Ka and basic target air quantity tQacb,from the expression tQac=tQacb×Ka.

Returning to step S2003, if the engine is operated out of the splitretard combustion mode, step S2009 occurs.

At step S2009, basic target air quantity tQacb itself is set as targetair quantity tQac, that is, tQac=tQacb.

After steps S2008 or S2009, step S2010 occurs.

At step S2010, the throttle opening of throttle valve 5 and the EGRvalve opening of EGR control valve 12 are controlled to bring the actualair quantity closer to target air quantity tQac. In more detail, first,throttle valve 5 is controlled in a manner so as to realize target airquantity tQac. Thereafter, for the purpose of fine adjustment, EGRcontrol valve 12 is feedback-controlled, so that actual air quantityQac, detected by airflow meter 23, is brought closer to target airquantity tQac.

Referring now to FIG. 26, there is shown the fuel injection quantitycontrol routine. The fuel injection quantity control routine of FIG. 26is also executed as time-triggered interrupt routines to be triggeredevery predetermined time intervals.

At step S2101, information concerning actual air quantity Qac, detectedby airflow meter 23, is read.

At step S2102, a target fuel injection quantity tQF is calculated ordetermined based on actual air quantity Qac and the desired λ.

At step S2103, each fuel injector 9 is controlled to bring the actualfuel injection quantity closer to target fuel injection quantity tQF.

As set forth above, during the split retard combustion period, targetair quantity tQac, which is determined based on the required enginetorque (equivalent to required fuel injection quantity QFDRV) and thedesired λ, is increasingly compensated for by final torque correctionfactor Ka (=Ka1×Ka2×Ka3). Additionally, target fuel injection quantitytQF is calculated based on actual air quantity Qac and the desired λ,and the actual fuel injection quantity is controlled to realize thetarget fuel injection quantity. Thus, it is possible to effectivelysuppress the engine torque drop, while realizing the desired λ.Additionally, the torque compensation is achieved, taking into accountthe main injection timing (see the map of FIG. 28), and therefore it ispossible to attain the optimum combustion control, while keeping theengine torque generated from engine 1 constant during the maincombustion period. This enhances the combustion stability and thedrivability.

Referring now to FIG. 30, there is shown the catalyst temperatureestimation routine used for estimation or prediction of the catalysttemperature of NOx trap catalyst 13. The catalyst temperature estimationroutine of FIG. 30 is also executed as time-triggered interrupt routinesto be triggered every predetermined time intervals. As describedpreviously, the catalyst temperature of NOx trap catalyst 13 isestimated based on an exhaust temperature Te1, which is detected byupstream exhaust temperature sensor (1^(st) exhaust temperature sensor)29 _(T1) and sensed just downstream of NOx trap catalyst 13. Forcatalyst temperature prediction, a predetermined catalyst-temperaturebasic predicted value TCbase versus exhaust temperature Te1characteristic model (or a predetermined catalyst-temperature basicpredicted value TCbase versus exhaust temperature Te1 characteristicmap) is preprogrammed, taking account of a heat capacity (a mass) and aheat conductivity of NOx trap catalyst 13, and a heat loss occurringbetween NOx trap catalyst 13 and 1^(st) exhaust temperature sensor 29_(T1) The catalyst temperature, exactly, catalyst-temperature basicpredicted value TCbase, is predicted or estimated or retrieved based on(i) exhaust temperature Te1, detected by 1^(st) exhaust temperaturesensor 29 _(T1) and (ii) a time rate of change ΔTe1 of exhausttemperature Te1, from the preprogrammed catalyst-temperature basicpredicted value TCbase versus exhaust temperature Te1 characteristicmodel.

At step S3001, exhaust temperature Te1, detected by 1^(st) exhausttemperature sensor 29 _(T1) is read.

At step S3002, a time rate of change ΔTe1 of exhaust temperature Te1 isarithmetically calculated by way of differentiating exhaust temperatureTe1.

At step S3003, basic predicted value TCbase of the catalyst temperatureof NOx trap catalyst 13 is estimated or retrieved based on (i) exhausttemperature Te1 and (ii) its time rate of change ΔTe1, from thepreprogrammed catalyst-temperature basic predicted value TCbase versusexhaust temperature Te1 characteristic model.

At step S3004, an exhaust gas flow rate is calculated or retrieved basedon target air quantity tQac and target fuel injection quantity tQF, froma preprogrammed map (not shown).

At step S3005, a first-order lag gain D (a rounding process gain) iscalculated based on the exhaust gas flow rate. Concretely, the smallerthe exhaust gas flow rate, the greater the first-order lag gain D.

At step S3006, a final catalyst-temperature predicted value TC iscalculated by making the first-order lag processing to basic predictedvalue TCbase, based on first-order lag gain D. Concretely, by way of theweighted mean processing for the previous value TCn−1 of finalcatalyst-temperature predicted value TC and the current value of basicpredicted value TCbase, final catalyst-temperature predicted value TC iscalculated, as follows.TC=TCn−1×D+TCbase×(1−D)where TCn−1 denotes the previous value of final catalyst-temperaturepredicted value TC, TCbase denotes the current value of basic predictedvalue TCbase, first-order lag gain D is used as a weight of the weightedmean processing and set within a predetermined range defined by aninequality 0<D<1, the previous value TCn−1 of final catalyst-temperaturepredicted value TC has a weight D, and the current value of basicpredicted value TCbase has a weight (1−D).

As discussed above, first-order lag gain D (the weight of previous valueTCn−1 of final catalyst-temperature predicted value TC) is set toincrease, as the exhaust gas flow rate decreases. Therefore, the timerate of change of final catalyst-temperature predicted value TC tends todecrease, as the exhaust gas flow rate reduces. As appreciated, duringoperation of the engine at a small exhaust gas flow rate, as a matter ofcourse, the amount of exhaust gas functioning to remove heat from NOxtrap catalyst 13 is also small. The small exhaust gas flow rate meansthe difficulty of removing heat (heat of reaction of oxidation) from NOxtrap catalyst 13. That is, there is a phenomenon that the Nox trapcatalyst temperature itself does not fall but may be kept almostconstant, even when the exhaust temperature Te1 detected downstream ofNOx trap catalyst 13 tends to fall. The previously-discussed relationalexpression of final catalyst-temperature predicted value TC based onfirst-order lag gain D is advantageous with respect to accuratesimulation or prediction of the previously-noted phenomenon.

Referring now to FIG. 31, there is shown the DPF temperature estimationroutine used for estimation or prediction of the DPF temperature of DPF14. The DPF temperature estimation routine of FIG. 31 is also executedas time-triggered interrupt routines to be triggered every predeterminedtime intervals. As described previously, the DPF temperature isestimated based on exhaust temperatures Te1 and Te2, respectivelydetected by 1^(st) and 2^(nd) exhaust temperature sensors 29 _(T1) and29 _(T2), located upstream and downstream of DPF 14. As appreciated,generally, the heat capacity of DPF 14 is greater than that of NOx trapcatalyst 13. Owing to the greater heat capacity, the rate of temperaturechange of DPF 14 with respect to a change in exhaust temperature isrelatively moderate. By way of the use of both of the detected valuesTe1 and Te2 of upstream and downstream exhaust temperature sensors 29_(T1) and 29 _(T2), it is possible to simply easily estimate the DPFtemperature (see the only two steps S4001 and S4002 of FIG. 31).

At step S4001, exhaust temperatures Te1 and Te2, detected by 1^(st) and2^(nd) exhaust temperature sensors 29 _(T1) and 29 _(T2), are read.

At step S4002, a DPF temperature predicted value Tdpf is calculatedbased on exhaust temperatures Te1 and Te2 from the predeterminedweighted-mean processing defined by the following expression.Tdpf=Te 1×W+Te 2×(1−W)where Te1 and Te2 respectively denote the exhaust temperature detectedby 1^(st) exhaust temperature sensor 29 _(T1) located upstream of DPF 14and the exhaust temperature detected by 2^(nd) exhaust temperaturesensor 29 _(T2) located downstream of DPF 14, and W denotes aweighted-mean constant and is set within a predetermined range definedby an inequality 0<W<1.

1^(st) exhaust temperature Te1 has the weight W, while 2^(nd) exhausttemperature Te2 has the weight (1−W). In due consideration of therelatively greater heat capacity of DPF 14, the weight (W) of 1^(st)exhaust temperature Te1 is set to be higher than the weight (1−W) of2^(nd) exhaust temperature Te2. For instance, in the system of the shownembodiment, the weight (W) of 1^(st) exhaust temperature Te1 is set to0.7 (that is, W=0.7). In other words, the weight of (1−W) of 2^(nd)exhaust temperature Te2 is set to 0.3 (that is, (1−W)=0.3).

As will be appreciated from the above, according to the system of theembodiment, in presence of a request of an exhaust temperature rise,based on an operating condition of the exhaust purifying device,including at least NOx trap catalyst 13 and DPF 14, thepreviously-discussed split retard combustion mode is initiated. At thesame time, the temperature of the exhaust purifying device (NOx trapcatalyst 13 and DPF 14) is estimated or predicted. When the predictedtemperature value of the exhaust purifying device exceeds itspredetermined temperature threshold value, that is, upper limit T22 ofthe desired DPF temperature during DPF regeneration (see S103 in FIG.3), or upper limit T42 of the desired catalyst temperature during SOxregeneration (see S203 in FIG. 4), the system of the embodiment executesthe fail-safe processing that the injection timing (the start tm1 ofmain fuel injection in FIG. 15A) of main fuel injection for maincombustion is compensated for in the timing-advance direction (see theflow from step S103 through steps S111-S113 to step S114 in FIG. 3 andsee the flow from step S203 through steps S211-S213 to step S214 in FIG.4). Therefore, by virtue of such a fail-safe function (the phase-advanceprocessing of the main fuel injection timing), it is possible to rapidlytimely lower the exhaust temperature and consequently to protect theexhaust purifying device (NOx trap catalyst 13 and DPF 14), even whenthe undesirably excessive temperature rise of the exhaust purifyingdevice occurs owing to certain causes.

When executing the fail-safe processing for the exhaust purifyingdevice, the feedback compensation amount (phase-advanced amounttadvance) of the injection timing of main fuel injection for maincombustion is calculated based on a temperature deviation ΔT(ΔT=Tdpf−T22 during DPF regeneration, or ΔT=TC−T42 during sulfurpoisoning release) between the predicted temperature value and thepredetermined temperature threshold value. The feedback gain G ofexhaust temperature feedback control is calculated based on temperaturedeviation ΔT (ΔT=Tdpf−T22 during DPF regeneration, or ΔT=TC−T42 duringsulfur poisoning release). Thus, feedback gain G is variably adjusteddepending on temperature deviation ΔT. Thus, when temperature deviationΔT is large, feedback gain G is increasingly compensated for, so as torapidly lower the exhaust temperature. On the contrary, when temperaturedeviation ΔT is small, feedback gain G is decreasingly compensated forso as to prevent undesirable hunting (overshoot and/or undershoot),occurring due to a delay in exhaust temperature change based on fuelinjection timing control. Compensating for feedback gain G based ontemperature deviation ΔT effectively enhances the convergence of theexhaust purifying device temperature to the desired temperature value.

In estimating or predicting the exhaust purifying device temperature, inparticular, NOx trap catalyst temperature (TC), based on the exhausttemperature Te1 detected by 1^(st) exhaust temperature sensor 29 _(T1)located downstream of the exhaust purifying device (NOx trap catalyst13), the system of the embodiment utilizes a predetermined exhaustpurifying device predicted temperature versus exhaust temperature (Te1)characteristic model defining the correlation between the exhaustpurifying device temperature and exhaust temperature Te1, based on aheat capacity and/or a heat conductivity of the exhaust purifyingdevice. Thus, it is possible to more accurately estimate or predict theexhaust purifying device temperature (NOx trap catalyst temperature TC).

Additionally, the exhaust purifying device temperature, estimated by thepredetermined characteristic model, is set or determined as a basicpredicted value TCbase of the exhaust purifying device temperature. Afinal exhaust-purifying-device-temperature predicted value TC iscalculated by making the first-order lag processing to the basicpredicted value TCbase, based on a first-order lag gain D. First-orderlag gain D is variably adjusted, changed or compensated for depending onan exhaust gas flow rate. Thus, it is possible to avoid or minimize theoccurrence of temperature prediction error, occurring owing to thedifficulty of removing heat from the exhaust purifying device duringengine operation at the small exhaust gas flow rate, thereby moregreatly enhancing the accuracy of temperature prediction.

In estimating or predicting the exhaust purifying device temperature, inparticular, DPF temperature (Tdpf), based on the exhaust temperaturesTe1 and Te2 detected by 1^(st) and 2^(nd) exhaust temperature sensors 29_(T1) and 29 _(T2) located upstream and downstream of the exhaustpurifying device (DPF 14), the system of the embodiment utilizes thesimple weighted-mean processing (Tdpf=Te1×W+Te2×(1−W)) for both of thesetwo exhaust temperatures Te1 and Te2. Thus, it is possible to simplyprecisely estimate or predict the exhaust purifying device temperature(DPF temperature Tdpf).

Furthermore, when the exhaust purifying device is comprised of both ofNOx trap catalyst 13 and DPF 14 located downstream of NOx trap catalyst13, 1^(st) exhaust temperature sensor 29 _(T1) is disposed downstream ofNOx trap catalyst 13 and upstream of DPF 14, whereas 2^(nd) exhausttemperature sensor 29 _(T2) is disposed downstream of DPF 14. Thecatalyst temperature TC of NOx trap catalyst 13 is estimated based onthe exhaust temperature Te1, detected by 1^(st) exhaust temperaturesensor 29 _(T1) disposed downstream of NOx trap catalyst 13 and upstreamof DPF 14. On the other hand, the DPF temperature Tdpf of DPF 14 isestimated based on the exhaust temperatures Te1 and Te2, detected by1^(st) and 2^(nd) exhaust temperature sensors 29 _(T1) and 29 _(T2)disposed upstream and downstream of DPF 14. By the use of only twoexhaust temperature sensors 29 _(T1) and 29 _(T2), it is possible toprecisely estimate or predict the catalyst temperature TC and the DPFtemperature Tdpf.

In the system of the shown embodiment, NOx trap catalyst 13 is laid outupstream of DPF 14. In other words, DPF 14 is laid out downstream of NOxtrap catalyst 13. Conversely, DPF 14 may be laid out upstream of NOxtrap catalyst 13. In such a case, the DPF temperature of DPF 14 isestimated based on the exhaust temperature Te1 detected by 1^(st)exhaust temperature sensor 29 _(T1) disposed downstream of DPF 14 andupstream of NOx trap catalyst 13. On the other hand, the catalysttemperature of NOx trap catalyst 13 is estimated based on the exhausttemperatures Te1 and Te2, detected by 1^(st) and 2^(nd) exhausttemperature sensors 29 _(T1) and 29 _(T2) disposed upstream anddownstream of NOx trap catalyst 13.

In the system of the shown embodiment, NOx trap catalyst 13 and DPF 14are constructed separately from each other. In lieu thereof, DPF 14 maybe constructed to be equipped integral with NOx trap catalyst 13. Thetemperature of the NOx-trap-catalyst-equipped DPF may be estimated basedon an exhaust temperature detected by an exhaust temperature locateddownstream of the NOx-trap-catalyst-equipped DPF.

As set out above, according to the system of the shown embodiment,during the DPF regeneration execution cycle, the system estimates theDPF temperature, and executes the fail-safe processing by way of thephase-advance of the main injection timing, when the estimated DPFtemperature rises excessively (Tdpf>T22). Likewise, during the sulfurpoisoning release (SOx regeneration) execution cycle, the systemestimates the catalyst temperature, and executes the fail-safeprocessing by way of the phase-advance of the main injection timing,when the estimated catalyst temperature rises excessively (TC>T42). Ifthe heat capacity of NOx trap catalyst 13 is remarkably relativelysmaller than that of DPF 14 and there is a problem of the deterioratedheat resisting property of NOx trap catalyst 13, it is preferable tocompare the catalyst temperature to the predeterminedcatalyst-temperature threshold value even during the DPF regenerationexecution cycle. If the estimated catalyst temperature rises excessively(TC>T42) during the DPF regeneration, the fail-safe processing must beinitiated by advancing the main injection timing, thereby effectivelypreventing the NOx trap catalyst from being heat-damaged even during theDPF regeneration cyce.

The entire contents of Japanese Patent Application No. 2003-282360(filed Jul. 30, 2003) are incorporated herein by reference.

While the foregoing is a description of the preferred embodimentscarried out the invention, it will be understood that the invention isnot limited to the particular embodiments shown and described herein,but that various changes and modifications may be made without departingfrom the scope or spirit of this invention as defined by the followingclaims.

1. A combustion control system of an internal combustion engineemploying an exhaust purifying device in an exhaust passage, comprising:sensors that detect operating conditions of the engine; a control unitbeing configured to be electronically connected to the sensors, forcombustion control and fail-safe purposes; the control unit comprising aprocessor programmed to perform the following, (a) estimating anoperating condition of the exhaust purifying device; (b) determining,based on the operating condition of the exhaust purifying device,whether a request for a rise in an exhaust temperature is present; (c)executing, by way of fuel injection control in presence of the requestfor the exhaust temperature rise, a split retard combustion mode inwhich a main combustion needed to produce a main engine torque and atleast one preliminary combustion occurring prior to the main combustionare both achieved and additionally the preliminary combustion takesplace near top dead center on a compression stroke and additionally themain combustion initiates after the preliminary combustion has beencompleted; (d) predicting a temperature of the exhaust purifying deviceto determine a predicted temperature value; and (e) executing afail-safe process according to which an injection timing of main fuelinjection for the main combustion is compensated for in a timing-advancedirection, when the predicted temperature value exceeds a predeterminedtemperature threshold value.
 2. The combustion control system as claimedin claim 1, wherein the processor is further programmed for: (f)calculating a feedback compensation amount for compensation for theinjection timing of main fuel injection in the timing-advance direction,based on a temperature deviation between the predicted temperature valueand the predetermined temperature threshold value; and (g) variablyadjusting a feedback gain for calculation of the feedback compensationamount, depending on the temperature deviation.
 3. The combustioncontrol system as claimed in claim 1, wherein the processor is furtherprogrammed for: (h) retrieving the temperature of the exhaust purifyingdevice based on an exhaust temperature detected downstream of theexhaust purifying device from a predetermined characteristic modeldefining a correlation between the temperature of the exhaust purifyingdevice and the exhaust temperature detected downstream of the exhaustpurifying device, based on at least one of a heat capacity and a heatconductivity of the exhaust purifying device, to determine the predictedtemperature value.
 4. The combustion control system as claimed in claim3, wherein the processor is further programmed for: (i) determining thetemperature of the exhaust purifying device, retrieved based on theexhaust temperature detected downstream of the exhaust purifying devicefrom the predetermined characteristic model, as a basic predicted valueof the temperature of the exhaust purifying device; (j) calculating afinal exhaust-purifying-device-temperature predicted value by making afirst-order lag process to the basic predicted value, based on afirst-order lag gain; and (k) variably adjusting the first-order laggain based on an exhaust gas flow rate.
 5. The combustion control systemas claimed in claim 1, wherein the processor is further programmed for:(l) predicting the temperature of the exhaust purifying device based onexhaust temperatures detected upstream and downstream of the exhaustpurifying device, by way of a weighted-mean process for both of theexhaust temperatures.
 6. The combustion control system as claimed inclaim 1, wherein: an injection quantity of preliminary fuel injectionfor the preliminary combustion is set to a fuel injection quantityneeded in order for an in-cylinder temperature obtained during a mainfuel injection period for the main combustion to exceed a self-ignitabletemperature value.
 7. The combustion control system as claimed in claim1, wherein: a start of the main combustion is retarded from a start ofthe preliminary combustion by at least 20 degrees of crankangle, forinitiating the main combustion after completion of the preliminarycombustion.
 8. The combustion control system as claimed in claim 1,wherein: an end of the main combustion is retarded by at least 50degrees of crankangle from the top dead center on the compressionstroke.
 9. The combustion control system as claimed in claim 1, wherein:the exhaust temperature is controlled by changing the injection timingof main fuel injection during the main combustion.
 10. The combustioncontrol system as claimed in claim 1, wherein the processor is furtherprogrammed for: (m) executing a torque compensation process that keeps atorque generated by the engine constant during the main combustion forminimizing an engine torque difference before and after switching to thesplit retard combustion mode.
 11. The combustion control system asclaimed in claim 1, wherein: the exhaust purifying device comprises aparticulate filter that accumulates particulate matter (PM) contained inexhaust gases, and a period that the request for the exhaust temperaturerise based on the operating condition of the exhaust purifying device ispresent, is at least a particulate-filter regeneration period duringwhich the PM accumulated in the particulate filter is burned and removedfrom the particulate filter by rising up the exhaust temperature. 12.The combustion control system as claimed in claim 1, wherein: theexhaust purifying device comprises a NOx trap catalyst that trapsnitrogen oxides contained in exhaust gases when an exhaust air-fuelmixture ratio is lean, and a period that the request for the exhausttemperature rise based on the operating condition of the exhaustpurifying device is present, is at least a sulfur poisoning releaseperiod during which sulfur oxides trapped by the NOx trap catalyst isdesorbed from the NOx trap catalyst by rising up the exhausttemperature.
 13. The combustion control system as claimed in claim 1,wherein: the exhaust purifying device comprises (i) a NOx trap catalystthat traps nitrogen oxides contained in exhaust gases when an exhaustair-fuel mixture ratio is lean, and (ii) a particulate filter thataccumulates particulate matter contained in the exhaust gases and isdisposed downstream of the NOx trap catalyst, and which furthercomprises a first exhaust temperature sensor disposed downstream of theNOx trap catalyst and upstream of the particulate filter, and a secondexhaust temperature sensor disposed downstream of the particulatefilter, and wherein a temperature of the NOx trap catalyst is predictedbased on an exhaust temperature detected by the first exhausttemperature sensor disposed downstream of the NOx trap catalyst, and atemperature of the particulate filter is predicted based on both of theexhaust temperature detected by the first exhaust temperature sensor andan exhaust temperature detected by the second exhaust temperature sensordisposed downstream of the particulate filter.
 14. The combustioncontrol system as claimed in claim 13, wherein the processor is furtherprogrammed for: (n) calculating a temperature deviation between theNOx-trap-catalyst temperature predicted and the predeterminedtemperature threshold value; (o) variably adjusting a feedback gain of afeedback compensation amount for compensation for the injection timingof main fuel injection in the timing-advance direction, depending on thetemperature deviation; (p) calculating the feedback compensation amountby multiplying the temperature deviation by the feedback gain; and (q)advancing the injection timing of main fuel injection by the feedbackcompensation amount, when the NOx-trap-catalyst temperature predictedbased on the exhaust temperature detected by the first exhausttemperature sensor exceeds the predetermined temperature thresholdvalue.
 15. The combustion control system as claimed in claim 13, whereinthe processor is further programmed for: (n) calculating a temperaturedeviation between the particulate-filter temperature predicted and thepredetermined temperature threshold value; (o) variably adjusting afeedback gain of a feedback compensation amount for compensation for theinjection timing of main fuel injection in the timing-advance direction,depending on the temperature deviation; (p) calculating the feedbackcompensation amount by multiplying the temperature deviation by thefeedback gain; and (q) advancing the injection timing of main fuelinjection by the feedback compensation amount, when theparticulate-matter temperature predicted based on the exhausttemperatures detected by the first and second exhaust temperaturesensors exceeds the predetermined temperature threshold value.
 16. Acombustion control system of an internal combustion engine employing anexhaust purifying device in an exhaust passage, comprising: sensor meansfor detecting operating conditions of the engine; a control unit beingconfigured to be electronically connected to the sensor means, forcombustion control and fail-safe purposes; the control unit comprising:(a) means for estimating an operating condition of the exhaust purifyingdevice; (b) means for determining, based on the operating condition ofthe exhaust purifying device, whether a request for a rise in an exhausttemperature is present; (c) means for executing, by way of fuelinjection control in presence of the request for the exhaust temperaturerise, a split retard combustion mode in which a main combustion neededto produce a main engine torque and at least one preliminary combustionoccurring prior to the main combustion are both achieved andadditionally the preliminary combustion takes place near top dead centeron a compression stroke and additionally the main combustion initiatesafter the preliminary combustion has been completed; (d) means forpredicting a temperature of the exhaust purifying device to determine apredicted temperature value; and (e) means for executing a fail-safeprocess according to which an injection timing of main fuel injectionfor the main combustion is compensated for in a timing-advancedirection, when the predicted temperature value exceeds a predeterminedtemperature threshold value.
 17. A method of executing a fail-safefunction for an exhaust purifying device disposed in an exhaust passageof an internal combustion engine capable of recovering an operatingcondition of the exhaust purifying device, the method comprising:estimating the operating condition of the exhaust purifying device;determining, based on the operating condition of the exhaust purifyingdevice, whether a request for a rise in an exhaust temperature ispresent; executing, by way of fuel injection control in presence of therequest for the exhaust temperature rise, a split retard combustion modein which a main combustion needed to produce a main engine torque and atleast one preliminary combustion occurring prior to the main combustionare both achieved and additionally the preliminary combustion takesplace near top dead center on a compression stroke and additionally themain combustion initiates after the preliminary combustion has beencompleted; predicting a temperature of the exhaust purifying device todetermine a predicted temperature value; and executing a fail-safeprocess according to which an injection timing of main fuel injectionfor the main combustion is compensated for in a timing-advancedirection, when the predicted temperature value exceeds a predeterminedtemperature threshold value.
 18. A method of executing a fail-safefunction for an exhaust purifying device including (i) a NOx trapcatalyst that traps nitrogen oxides contained in exhaust gases when anexhaust air-fuel mixture ratio is lean, and (ii) a particulate filterthat accumulates particulate matter contained in the exhaust gases andis disposed downstream of the NOx trap catalyst, both disposed in anexhaust passage of an internal combustion engine capable of recoveringan operating condition of the exhaust purifying device, the methodcomprising: disposing a first exhaust temperature sensor downstream ofthe NOx trap catalyst and upstream of the particulate filter; disposinga second exhaust temperature sensor downstream of the particulatefilter, predicting a temperature of the NOx trap catalyst based on anexhaust temperature detected by the first exhaust temperature sensor;predicting a temperature of the particulate filter based on both of theexhaust temperature detected by the first exhaust temperature sensor andan exhaust temperature detected by the second exhaust temperaturesensor; estimating the operating condition of the exhaust purifyingdevice; determining, based on the operating condition of the exhaustpurifying device, whether a request for a rise in an exhaust temperatureis present; executing, by way of fuel injection control in presence ofthe request for the exhaust temperature rise, a split retard combustionmode in which a main combustion needed to produce a main engine torqueand at least one preliminary combustion occurring prior to the maincombustion are both achieved and additionally the preliminary combustiontakes place near top dead center on a compression stroke andadditionally the main combustion initiates after the preliminarycombustion has been completed; and executing a fail-safe processaccording to which an injection timing of main fuel injection for themain combustion is phase-advanced, when the at least one of theNOx-trap-catalyst temperature predicted and the particulate-filtertemperature predicted exceeds a predetermined temperature thresholdvalue.
 19. The method as claimed in claim 18, further comprising:calculating a temperature deviation between the NOx-trap-catalysttemperature predicted and the predetermined temperature threshold value;variably adjusting a feedback gain of a feedback compensation amount forphase-advance of the injection timing of main fuel injection, dependingon the temperature deviation; calculating the feedback compensationamount by multiplying the temperature deviation by the feedback gain;and phase-advancing the injection timing of main fuel injection by thefeedback compensation amount, when the NOx-trap-catalyst temperaturepredicted based on the exhaust temperature detected by the first exhausttemperature sensor exceeds the predetermined temperature thresholdvalue.
 20. The method as claimed in claim 18, further comprising:calculating a temperature deviation between the particulate-filtertemperature predicted and the predetermined temperature threshold value;variably adjusting a feedback gain of a feedback compensation amount forphase-advance of the injection timing of main fuel injection, dependingon the temperature deviation; calculating the feedback compensationamount by multiplying the temperature deviation by the feedback gain;and phase-advancing the injection timing of main fuel injection by thefeedback compensation amount, when the particulate-matter temperaturepredicted based on the exhaust temperatures detected by the first andsecond exhaust temperature sensors exceeds the predetermined temperaturethreshold value.